Allow subscription to a forum

When subscribed to a forum, the user will be notified of all new and
edited comments.
Fix brainfart.
Add tests for forum subscription.

Also change 'subscribe' to show when we're subscribed to a forum.
Don't include derived files.
Fix subscribe template logic

but damn it's ugly.
Adding mail-in to forum. (Work in progress.)
Fix syntax
Mail-in functionality (WIP)
Add smtp2zope as an optional extra.
Back out mailin from here, moved to mailin.py
Moved to mailin.py
Allow reply to notification mails.
Remove pdb
Fix template condition for forum subscription
Make notification texts distinguishable
Add create_plaintext_message to strip HTML from notification
Mailin should go to the board, not a specific forum

Otherwise you need to setup smtp2zope for every forum.
Resolve paths relative to the board

Necessary in case of virtual hosting.
Deal with multipart and encoded messages
Format mailed comments as preformatted for adding to forum