lib: Grant SELECT on auth_user

lib: Grant SELECT on auth_user

If a mail arrives with the 'X-Patchwork-Delegate' hint header, the
'patchwork.parser' script will need to index the users table to find the
appropriate user. This should be okay from a security perspective since
passwords are hashed and salted and the rest of the information is
mostly accessible publicly via the web UI and REST API.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Suggested-by: Ali Alnubani <alialnu@mellanox.com>
Closes: #365
(cherry picked from commit e69a2adcf50b57980d5eb0074cc72698d5cac31a)