fix django 1.11 migrations

[ltalirz]

• update django settings (cleanup + adaptation to django 1.11) together with @giovannipizzi
• fix migration of UUID column for databases created with previous schema versions
• make sure we give unicode strings to django (not byte strings)
• add test to check that uuid migration does not change uuids
• add test to check that no django schema migrations remain

Will look into adding sqlalchemy unique constraints on uuid column in a separate PR

[coveralls]

Coverage increased (+0.03%) to 68.851% when pulling f43ea24 on ltalirz:try-charfield into 7e7f062 on aiidateam:provenance_redesign.

[sphuber]

Will this fix the origin of the tests failing now in Django? Get a lot of errors in SQL where its trying to compare uuid types to varchars, telling me we should cast the values. And if that is so, how did that pass the checks before merging?

[ltalirz]

Get a lot of errors in SQL where its trying to compare uuid types to varchars, telling me we should cast the values.

Are you talking about tests on travis or tests run locally?
Locally, yes: if you did not create your database from scratch, then this would happen (sorry for this; was the first time for me editing the db schema). Merging the PR will not fix this, since the (flawed) migration has already been applied (you'll need to create a new test database)
On travis: I would not understand why you get problems there...

[sphuber]

Ah yeah you are right, that was the problem. After dropping and recreating the database it is fine