[Collideoscope] Fix report sending to always use Email method

Overrides `_fallback_body_sender` from the UK cobrand so that we don't do things
differently for bodies in London or Northern Ireland.

Note: Northern Ireland reports will actually be sent to 1 of 2 (or maybe 4?)
highways agencies, but we've assigned the appropriate email addresses to each
NI council body in the database, so we don't need to use the special NI
sending method to achieve this.

Closes mysociety/FixMyStreet-Commercial#623