Extensions to `@admin.register` fixer

Description

Following #182 / #189 , there are a few more cases that could be covered with further work.

1. Support kwarg form ✅ #192

[e-mail](MyModel)
 class MyModelAdmin(...):
    ...

-admin.site.register(MyModel, admin_class=MyModelAdmin)

1. Support multiple models ✅ #200

[e-mail](Model1, Model2)
 class MyModelAdmin(...):
    ...

-admin.site.register(Model1, MyModelAdmin)
-admin.site.register(Model2, MyModelAdmin)

[e-mail](Model1, Model2)
 class MyModelAdmin(...):
    ...

-admin.site.register((Model1, Model2), MyModelAdmin)

1. Support when there are other class decorators, insert at top ✅ #193

[e-mail](MyModel)
 @something
 class MyModelAdmin(...):
    ...

-admin.site.register(MyModel, MyModelAdmin)

1. Support custom admin sites ✅ #228

from myapp.admin import custom_site

[e-mail](MyModel, site=custom_site)
 class MyModelAdmin(...):
    ...

-custom_site.register(MyModel, MyModelAdmin)

• detect custom site objects heuristically, to avoid affecting other code that uses a ‘register’ pattern, perhaps when the object’s name ends in ‘site’, and the registered class name ends in ‘Admin’ (and maybe the file looks like an admin file, called “admin.py” or in an “admin” directory?)

1. Also work for from django.contrib.gis import admin - ✅ #204 This imports the register decorator for convenience when authoring GIS apps.

[adamchainz]

Alright we’ve checked everything off from this list! 🥳

This is now a really awesome feature that covers many cases. Great work @UnknownPlatypus .

[UnknownPlatypus]

@adamchainz Absolutely, would be nice to work on that and I think I have some ideas on how to tackle them.