nixos/gitlab: Use postgresql module options to provision local db

Use the postgresql module to provision a local db (if
databaseCreateLocally is true) instead of doing this locally.

Switch to using the local unix socket for db connections by default;
this is needed since dbs created by the postgresql module only support
peer authentication.

Instead of running the rake tasks db:schema:load, db:migrate and
db:seed_fu, run gitlab:db:configure, which in turn runs these tasks
when needed.

Solves issue #53852 for gitlab.