fix: delete user saved selections if user is gone

[f-necas]

Sometimes, user synchronization was failing with the error:

Caused by: org.postgresql.util.PSQLException: ERROR: update or delete on table "users" violates foreign key constraint "fkq971r8i90nbrn42wgw43f71bd" on table "usersavedselections"                                       │
│   Detail: Key (id)=(401) is still referenced from table "usersavedselections".

It's because someone who doesn't exist anymore has used the "Favorite" functionnality of GN (often used with datahub)

So we now delete those selections before trying to delete user.

[fvanderbiest]

Nice catch.
No side effects expected ?

[f-necas]

I don't think so, user should be in sync with console and external_user table and it happens only on them.

[pmauduit]

I thought I had commented on this one: I am surprised it requires an explicit deleteAllByUser call, and would have expected that JPA was capable of removing linked objects in its own model.

Other than that, looks fine.