Simplify the declaration of custom field support in models

[jeremystretch]

Proposed Changes

Currently, a NetBox model must meet two conditions for custom fields to function properly:

1. Inherit from CustomFieldsMixin
2. Register as having custom fields via the extras_features() decorator

The second step is redundant, given that it should be assumed that any model inheriting from CustomFieldsMixin supports custom fields. However, failing to register via extras_features() excludes the model from the list of available models when creating a custom field, and from other such selections.

We can obviate the need to register the model explicitly by instead connecting a receiver function to Django's class_prepared signal and checking for custom field support on the class.

Justification

Eliminates redundant code and reduces the possibility for human error.

[jeremystretch]

A similar issue exists with the declaration of support for tags: A model must both inherit from TagsMixin (to receive the appropriate manager) and register via extras_features().

[jeremystretch]

This has been resolved under #8392.