Rather than try to fix up various bits where we search & all and
wonder what index we should be using, make the column a CIText.
For mergebot the main use case would be properly handling
delegate=XXX: currently if XXX is not a case-sensitive match we're
going to create a new partner with the new github login and
give *them* delegation, and the intended target of the delegation
isn't going to work correctly.
Also try to install the citext extension if it's not in the database,
and run the database-creation process with `check=True` so if that
fails we properly bubble up the error and don't try to run tests on a
corrupted / broken DB.
Fixes#318
