The Review Board administration UI has historically been built on top of

the Django admin UI, with a lot of customizations wrapping that

foundation. Review Board 4.0 has been moving away from the Django admin

UI, while retaining compatibility with it. Still, there are aspects of

it that we depend on, but need to customize, and the safest way to do

that is by using the support built into it rather than attempting to

work around it.
To help with that, this change introduces a new AdminSite of our own,

which has some information overridden and new functionality added.

AdminSite controls all the views and settings for the administration

UI, and Django normally provides one that we can use. By providing our

own, we're able to fix up some wording in places, and reuse some

capabilities of our normal Review Board login form when users try

logging into the admin UI.
While Django does allow for custom admin UIs, it doesn't really expect

the main one to be replaced. ModelAdmins are still generally

registered with the main one. Because of that, our AdminSite actually

shares all the same registration state as the main one. It's a hack, but

we gain enough to justify it.
We also provide a new ModelAdmin "base class." This is a very simple

subclass of the Django's own ModelAdmin, which currently provides a

documented field that can be overridden for the fieldset template

(something that was available in our new admin UI but not well

documented).
Going forward, all new ModelAdmin subclasses should use our new one as

the base class. We'll continue to support Django's version, but using

our own will lead to advantages in customizing behavior in the

administration UI.
This also gets rid of the model auto-discovery, which is no longer

needed.

All unit tests pass.
Tested the functionality of the administration UI to make sure things

are still working as expected.
Tested that the new login form works as expected.
Compared the list of available models in the database section with this

change and without it, making sure we didn't lose anything but disabling

the auto-discovery.