Add Flask config override

This has previously been done in Invenio-Theme-TUW; however, it logically fits better into Invenio-Config-TUW.

Because of interference (race conditions) with other Invenio modules with the blueprint.record_once approach, and because we don't really need the overrides in the invenio shell, we decided to revert the override mechanism back to 'app.before_first_request'.