Document restoring from backups.

MySQL does DROP IF EXISTS $table; CREATE TABLE $table; which makes this
straightforward (you need a database and a user).  In fact, you could do
make initdb and restore from your backup and be fine, but I erred on
doing the least needed.

Our postgres backups don't clean, and if they did, they would get weird
errors because we run two backups (sessions and the rest) and clean
wants to drop the public namespace and you get errors about sessions
existing.  So, we don't switch to clean.  Similar problems exist with
telling pg_dump to ignore rights/acls since an admin would need to rerun
acl.Pg for any extension that provides one (say assets).

Instead, come up with a way to work with the existing backups which just
CREATE TABLE; COPY and GRANT. This means you need a database and a user
but *cannot* have tables. etc/acl.Pg assumes tables exist and will
create a user and then fail to GRANT rights noisily. Rather than scare
the admin by telling them to run rt-setup-database --action create,acl
and ignore the errors, we'll create and then use createuser to set up
the user.  This calls out that RT needs a separate step for "create
user".  We should also allow rt-setup-database --action drop,create but,
that's another branch.

The instructions here should be sufficient to restore from the the
backups we've been documenting.
Merge branch '4.2/restoring-from-backups' into 4.2-trunk