Fixes #15698: Drop and recreate FK constraints on ObjectPermission M2M tables

[jeremystretch]

Fixes: #15698

Modifies the users.0008_flip_objectpermission_assignments migration to drop & recreate (rather than rename) FK constraints which may not exist in older databases.

[jeremystretch]

I should note that I compared the resulting schema between two upgrade paths:

• v3.7 -> branch
• v2.11 -> 3.7 -> branch

The resulting sets of constraints are identical across both affected tables.

[tobiasge]

My tests with this PR where successful. All migrations completed on our database:

Operations to perform:
  Apply all migrations: account, auth, circuits, contenttypes, core, dcim, django_rq, extras, ipam, sessions, social_django, taggit, tenancy, users, virtualization, vpn, wireless
Running migrations:
  Applying dcim.0186_location_facility... OK
  Applying extras.0108_convert_reports_to_scripts... OK
  Applying extras.0109_script_model... OK
  Applying extras.0110_remove_eventrule_action_parameters... OK
  Applying extras.0111_rename_content_types... OK
  Applying extras.0112_tag_update_object_types... OK
  Applying extras.0113_customfield_rename_object_type... OK
  Applying tenancy.0015_contactassignment_rename_content_type... OK
  Applying users.0005_alter_user_table... OK
  Applying users.0006_custom_group_model... OK
  Applying users.0007_objectpermission_update_object_types... OK
  Applying users.0008_flip_objectpermission_assignments... OK
  Applying vpn.0005_rename_indexes... OK

Thank you for the fix!

[a084ed22]

I confirm that this fixes the issue. Migrations apply cleanly now.

[jeremystretch]

Thank you both for the confirmation!