Ensure database indexes are created for all generic foreign keys

[jeremystretch]

Proposed Changes

Audit all NetBox models and ensure that appropriate PostgreSQL indexes have been created for the combination of content type and object ID wherever generic foreign keys are in use.

Justification

This was prompted by #11775, which identified the absence of an index for cached search values as a performance detractor. Per the Django docs:

Unlike for the ForeignKey, a database index is not automatically created on the GenericForeignKey, so it’s recommended that you use Meta.indexes to add your own multiple column index.