Support hiding migrations and models when not using the DatabaseStore

[tim-schilling]

Description

There's a reasonable request to avoid having to use migrations (and presumably have access to the models), when the toolbar is configured to not use a database store. This supports that in two ways:

1. Hack the migrations via settings.MIGRATION_MODULES to avoid the migrations from being discovered
2. Prevent the models from being loaded by the AppConfig

References: #2234

Checklist:

• I have added the relevant tests for this change.
• I have added an item to the Pending section of docs/changes.rst.

[matthiask]

I like it!

We still have the option to move the models into a separate app later if this back proves to be problematic.