[forms] closes #2437859 - Detect and prevent concurrent edition of the same entity. Add the timestamp of form generation to each entity's meta-information fields. On form validation, check that no concurrent change is overwritten and raises a ValidationError in case of concurrent change. A nicer handling with a message and a link to the new version of the entity would be a good thing...

[migration] make sure the repo knows about all constraint types Adding new constraint types is cumbersome. It's easy to forget to do it in a migration script. So sync the constraint types at the beginning of each migration. Closes #3724892

[migration] stop caching the mapping from constraint type name to eid It's not so frequent that a cache seems necessary, and we were not invalidating that cache when adding a new constraint type. Related to #3724892.