[FIX] mail: limit messages in chatters
[FIX] fields: selection, do not try to translate label if label is empty

 - translate with no source returns first translation for this field, whatever the source
 - performance
[IMP] [TEST] website_forum: security fixes + tests

- fixed voting, karma check could be avoided
- fixed posting comments, now correctly checking karma (not for
notifications)
- fixed bootstraping of users, now not allowed to ask questions by default;
added validation email that gives the first karma points required to
participate
- added tests