NPL-374 Add Mixin Support and make CustomObject class ABC

[arthanson]

Fixes: #374

This issue is for bookmark support as all the mixins are broken up into separate issues, however the root cause of most of these are the same and the fix needs to work in all cases so this PR addresses multiple mixins.

There was also a root issue where CustomObject class was not abstract so the CustomObjects were using multi-table inheritance. This PR makes it an ABC and fixes the view querysets as they can't directly reference this model anymore. Unfortunately this makes any previously created CO not migratable anymore so any database needs to be re-created.

TaggingMixin is special case as it has a custom through model which will cause naming collisions for TaggableManager - "ValueError: You can't have two TaggableManagers with the same through model.", so the mixin can't be on the ABC, but instead is dynamically added to the CO in get_model creating the through model with a unique name.

Note: For most of these to work it needs changes in NetBox in how ObjectTypeManager and ObjectChangeQuerySet works. For temporary testing can modify ObjectTypeManager to return all objects: return self.get_queryset()

Should also fix multi-object field deletion.

The following mixins should be addressed in this PR

• BookmarksMixin
• ChangeLoggingMixin - change logs will appear / work if modify ObjectChangeQuerySet in NetBox, however branching support isn't implemented (separate PR)
• CloningMixin
• CustomLinksMixin - needs header formatting (separate ticket)
• CustomValidationMixin
• ExportTemplatesMixin
• JournalingMixin
• NotificationsMixin - needs further tests
• EventRulesMixin
• TaggingMixin