Patch to possibly add threading headers to cyclestreets mails

include a References header to be the same as the reply_to address, which is a valid email address for that thread. Will do basic threading.

Ideally the message ID would be populated here, and would take the message-in-thread-number that comes in far higher up in the call change, and do <msg-number>-<threadid>@domain so we get predictable msgids, and can then add the References programatically.
Improve layout and wording on group admin page.
Link to the home page of the group (rather than site) in the group management tabs.
Set various page titles that use interpolations. Fixes #208
Improve language, as per #208
Allow admins to delete site comments.
Missing translations.
Merge branch 'staging'