[FIX] mail: mail_alias conditionally made unique on write

Don't try to make the alias unique if writing on the same id (this is
necessary in res_config views)
[FIX] website_forum: safer conditions in UI for blockng post buttons
[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