[sobjects] Drop custom unique constraint checks in user registration service This constraints (on unique CWUser's login and EmailAddress's login) are handled by the backend. This follows "[hooks/integrity] kill CheckUniqueHook". Extend test for the ValidationError along the way.

[test] Add a test for user registration unicity constraint on email

[test] Clarify user registration service tests * split permissions test appart * avoid usage of internal_cnx as this does not seem need

[test/hooks] pep8 and unittest.main instead in unittest_hooks.py

[hooks/integrity] kill CheckUniqueHook It's the backend's job. The noticeable consequence (as can be seen in tests updated here) is that error message are a bit less user-friendly.

[migration] drop identity_relation if it exists It has been generated erroneously for a while, then fixed but not properly deleted from old databases. Don't use IF EXISTS for sqlserver compat.

[migration] Reserialize all constraints since yams introduce new JSON serializing format So we may drop support for older format in yams at some point.

[migration] Add IF EXISTS on DROP CONSTRAINT problem encountered on migration of a legacy database

[schema sync] Rename index when an entity type is renamed So we may still find them later using their expected name, and avoid collisions. Closes #13822045

[c-c] Add a command to check database index List indices expected by the schema but not in the database and vice-versa Closes #13822027