Add Problem::get_cobrand_logged

Purloined from Hakim's Open311 refactor branch,
commit 47daa02420a4f5a4264c31efcf2a2e8611c2f23e.
Add link to FMS for councils to report page

When viewing a problem that was reported via a council cobrand
on a cobrand other than the one it was reported on, a link to
mysociety.org's FixMyStreet for Councils page is shown.