[Collideoscope] Set the fallback body sender to Email

Because Collideoscope is based off the UK cobrand it falls back to
using the special Northern Ireland and London sending methods if
the DB doesn't specify one. We don't want that, but we do potentially
want to keep the devolved councils cleverness, so I've overriden
_fallback_body_sender only.

Fixes FixMyStreet-Commercial/#623