Fix user id migrations

[natkam]

• Enable foreign keys in sqlite: add the event listener for engine connections (see the sqlalchemy docs. More about it below.
• Fix the issues with the user_id migration, and small inconsistencies in the downgrade functions for another migration
• Add more tests (I cherry-picked the commit added by @florian-jaeger in test: added schema check for all db migrations #309)

Regarding the pragma enabling FKs: we can only register this event listener when the db backend is sqlite, because this pragma is sqlite-specific. With PostgreSQL, executing the pragma will cause an error.
I register the event listener based on the value of the DATABASE_TYPE env. var. - it probably would be better to actually get the sqlalchemy engine, or the GraderService's db_url, and check the dialect there... But I haven't managed to get it to work nicely yet.
UPDATE: I've figured out a better way. Seems to work as expected.

One more complication came up when I added a test that checks that the event listener works.
In the test environment, DATABASE_TYPE is not set, even though an sqlite database is used. We cannot set it "globally" , because some tests use a PostgreSQL database - and again, they would fail if the event listener was enabled.

Also: At the moment the migration tests (any many more!) would fail if we just enable the pragma in them all, because many of the tests would cause IntegrityErrors. And we do not have on delete behaviour configured for models related to other models via FKs; we'd have to add CASCADE to them all, I guess.

[florian-jaeger]

please add this to a different file, since it is not a assignment handler test e.g. test_db.py or something like that

[natkam]

Done. And I've also fixed some integrity errors in other tests and in migrations.