Flip the many-to-many relationships for users & groups to object permissions

[jeremystretch]

Proposed Changes

There are currently many-to-many relationships from the ObjectPermission model to the User and Group models within the users app. These are both defined on the ObjectPermission model:

class ObjectPermission(models.Model):
    groups = models.ManyToManyField(
        to='users.Group',
        blank=True,
        related_name='object_permissions'
    )
    users = models.ManyToManyField(
        to=get_user_model(),
        blank=True,
        related_name='object_permissions'
    )

This proposal entails moving the primary M2M field definitions from ObjectPermission to the User and Group models, as such:

class Group(models.Model):
    object_permissions = models.ManyToManyField(
        to='users.ObjectPermission',
        blank=True,
        related_name='groups'
    )

class User(models.Model):
    object_permissions = models.ManyToManyField(
        to='users.ObjectPermission',
        blank=True,
        related_name='users'
    )

This is essentially a cosmetic change, as the related field names for all three models will remain the same. However, a migration will be required to effect the swap and renamed the intermediary tables accordingly. There may also be some side effects of the reversal that need to be handled; testing will be needed to identify them. (In the event any serious blockers arise, this initiative should probably be punted.)

Justification

Moving these relationships to the User & Group models feels more natural and less surprising than the current arrangement, which exists only because NetBox did not employ custom user or group models prior to v4.0.