Move all change logging resources from `extras` to `core`

[jeremystretch]

Proposed Changes

Move all resources related to change logging from the extras app to core. This will entail moving the following:

• The ObjectChange model
• Relevant views and API endpoints
• The event_tracking() context manager

Justification

Although originally introduced as an optional "extra" feature in NetBox, change logging quickly become a core function of the application as it matured. It no longer makes sense for it to live under extras.