How do I add a custom button next to a field in Django admin?

class BlahAdmin(admin.ModelAdmin):
    ... Whatever definitions ...

    def queryset(self, request):
        return Blah.objects.extra(select={'computed': "count(field_x)"})

class GalleryAdmin(admin.ModelAdmin):

    form = GalleryForm

    def __init__(self, model, admin_site):
        self.form.admin_site = admin_site 
        super(GalleryAdmin, self).__init__(model, admin_site)

class GalleryForm(ModelForm):

    photos = ThumbnailChoiceField(queryset=Photo.objects.all(), label='Photos', widget=MyWidget(), required=False)

    def __init__(self, *args, **kwargs):
        super(GalleryForm, self).__init__(*args, **kwargs)
        rel = ManyToOneRel(self.instance.photos.model, 'id') 
        self.fields['photos'].widget = RelatedFieldWidgetWrapper(self.fields['photos'].widget, rel, self.admin_site) 

class Position(models.Model):
    expiration_date = models.DateField()
    ...

class Position(models.Model):
    expiration_date = models.DateField()
    expiration_date.is_expired_filter = True
    ...

from django.contrib.admin.filterspecs import FilterSpec, DateFieldFilterSpec
from django.utils.translation import ugettext as _
from datetime import datetime, date
class ExpiredFilterSpec(DateFieldFilterSpec):
    """
    Adds filtering by future and previous values in the admin
    filter sidebar. Set the is_expired_filter filter in the model field
    attribute 'is_expired_filter'.
    my_model_field.is_expired_filter = True
    """
    def __init__(self, f, request, params, model, model_admin, **kwargs):
        super(ExpiredFilterSpec, self).__init__(f, request, params, model,
                                                model_admin, **kwargs)
        today = date.today()
        self.links = (
            (_('All'), {}),
            (_('Not Expired'), {'%s__lt' % self.field.name: str(today),
                   }),
            (_('Expired'), {'%s__gte' % self.field.name: str(today),
                    }))
    def title(self):
        return "Filter By Expiration Date"
# registering the filter
FilterSpec.filter_specs.insert(0, (lambda f: getattr(f, 'is_expired_filter', False),
                                   ExpiredFilterSpec))


class PositionAdmin(admin.ModelAdmin):
    list_filter = ['expiration_date']

class MyModelAdmin(admin.ModelAdmin):

    # A template for a customized change view:
    change_form_template = 'path/to/your/custom_change_form.html'

    def response_change(self, request, obj):
        opts = self.model._meta
        pk_value = obj._get_pk_val()
        preserved_filters = self.get_preserved_filters(request)

        if "_customaction" in request.POST:
            # handle the action on your obj
            redirect_url = reverse('admin:%s_%s_change' %
                               (opts.app_label, opts.model_name),
                               args=(pk_value,),
                               current_app=self.admin_site.name)
             redirect_url = add_preserved_filters({'preserved_filters': preserved_filters, 'opts': opts}, redirect_url)
             return HttpResponseRedirect(redirect_url)
        else:
             return super(MyModelAdmin, self).response_change(request, obj)

 {% if save_on_top %}{% block submit_buttons_top %}{% submit_row %}{% endblock %}{% endif %}

 {% if save_on_top %}{% block submit_buttons_top %}{% custom_submit_row %}{% endblock %}{% endif %}

 {% block submit_buttons_bottom %}{% submit_row %}{% endblock %}

@register.inclusion_tag('path/to/your/custom_submit_line.html', takes_context=True)
def custom_submit_row(context):
    """
    Displays the row of buttons for delete and save.
    """
    opts = context['opts']
    change = context['change']
    is_popup = context['is_popup']
    save_as = context['save_as']
    ctx = {
        'opts': opts,
        'show_delete_link': (
            not is_popup and context['has_delete_permission'] and
            change and context.get('show_delete', True)
        ),
        'show_save_as_new': not is_popup and change and save_as,
        'show_save_and_add_another': (
            context['has_add_permission'] and not is_popup and
            (not save_as or context['add'])
        ),
        'show_save_and_continue': not is_popup and context['has_change_permission'],
        'is_popup': is_popup,
        'show_save': True,
        'preserved_filters': context.get('preserved_filters'),
    }
    if context.get('original') is not None:
        ctx['original'] = context['original']
    return ctx

{% load i18n admin_urls %}
<div class="submit-row">
{% if show_save %}<input type="submit" value="{% trans 'Save' %}" class="default" name="_save" />{% endif %}
{% if show_delete_link %}
    {% url opts|admin_urlname:'delete' original.pk|admin_urlquote as delete_url %}
    <p class="deletelink-box"><a href="{% add_preserved_filters delete_url %}" class="deletelink">{% trans "Delete" %}</a></p>
{% endif %}
{% if show_save_as_new %}<input type="submit" value="{% trans 'Save as new' %}" name="_saveasnew" />{% endif %}
{% if show_save_and_add_another %}<input type="submit" value="{% trans 'Save and add another' %}" name="_addanother" />{% endif %}
{% if show_save_and_continue %}<input type="submit" value="{% trans 'Save and continue editing' %}" name="_continue" />{% endif %}

<input type="submit" value="{% trans 'Custom Action' %}"  name="_customaction" />

</div>

from django.utils.translation import gettext, gettext_lazy as _

class CustomUserAdmin(UserAdmin):
    add_form = CustomUserCreationForm
    form = CustomUserChangeForm
    model = CustomUser
    list_display = ['username', 'introduction',]
    fieldsets = (
        (None, {'fields': ('username', 'password')}),
        (_('Personal info'), {'fields': ('first_name', 'last_name', 'email', 'introduction')}),
        (_('Permissions'), {
            'fields': ('is_active', 'is_staff', 'is_superuser', 'groups', 'user_permissions'),
        }),
        (_('Important dates'), {'fields': ('last_login', 'date_joined')}),
    )