lib: Add update permissions to 'nobody' postgres user

lib: Add update permissions to 'nobody' postgres user

This user needs the ability to change some attributes of 'Patch' and
'CoverLetter' instances that are stored in the 'patchwork_submission'
table, since both are are concrete subclasses of 'Submission'.

Stable-only since the 'Submission model has been removed on 'master'.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Suggested-by: Ali Alnubani <alialnu@mellanox.com>
Closes: #364
Stable-Only