Use repository slug as Primary key instead of phabricator ID

[La0]

Followup of #1523

This is the same approach to generalize the backend without requiring Phabricator IDs.

We already have a uniqe slug on the Repository model, both phabricator IDs should be made nullable. All relations must be updated before-hand in a data migration.

All the endpoints must also be updated, and finally the bot too (as it references repositories during revision creation).

[La0]

@EvaBardou can start working on this.