Replace organization_users LFKs with regular FKs

The organization_users table uses LFKs for both its organizations and users table relationships. LFKs lack features provided by regular foreign keys, especially related to data integrity. Due to their asynchronous nature they can lead to problems.

We should convert these relationships to regular foreign keys.

Note we are still unsure how we will handle users in Cells 1.5 when they will need to be clusterwide and perhaps LFKs will be unavoidable. However for now in Cells 1.0 we should leverage the database tools we have available.

Status

We can work on this after %18.2 has been released. We need two subsequent MR's:

• Finalize the background migration. T
• Validate the foreign keys for user_id and organization_id

The background migration for fixing the data (if needed) did finish on gitlab.com /chatops run batched_background_migrations status 2000922

We should now Finalize the migration. According to the docs, this should happen after the last required stop. This means this finanalization should be deliverd in the %18.3 release