Review Board 4.0 introduced its own AdminSite class, which attempted

to mirror the main one. This mostly worked, but some apps (namely in our

case, Django Evolution) didn't make it into our mirror.
This is because we were actually sharing instances of the dictionaries,

expecting that they wouldn't be replaced (since AdminSite keeps them

around), but it turns out that admin.py discovery would copy and reset

the instance any time an admin.py was missing from a module. That

broke our shared instances, meaning that we could only share up to the

first module missing an admin.py.
We now copy state, rather than attempting to share state, and override

the AdminSite instance in django.contrib.admin. This isn't ideal,

but Django, despite supporting custom AdminSite subclasses, doesn't

offer an alternative (at least in 1.11). This is the approach other

projects appear to have settled on.

Tested loading all parts of the admin UI (list and change forms),

without encountering any crashes.