[IMP] fields: infer required=True on related fields

One can infer related=True on a non-stored related field if all fields on the
path are related.  This cannot be done if the related field is stored: when you
create a record, the database row is created first, and the related field is
computed and stored afterwards.  Making the field required in that case would
trigger a non-null constraint violation.
[IMP] fields: remove unused imports
[IMP] website_mail: actually render the html body of the template instead of escaping it
[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
[FIX] remove wrong filter (account)

For some reason, a filter was made on the non-existing field 'categ_id'.
Result: crash crash boom.