Rearchitect the logic for registering models & model features

[jeremystretch]

Proposed Changes

Currently, NetBox automatically registers all models and their features by hooking into Django's class_prepared signal. This issue proposes moving the registration triggers into the ready() method of the AppConfig class for each app.

Justification

Although the current approach has generally worked well, issues arose while introducing a custom user model under #12795. Because Django must load the (now custom) user model before the signal handler, we must reorder the logic. This will also enable cleaner separation of functions.