Improvements on translations page.

This seems to lead to various troubles as save can be called at syncdb
time, when the database layer is not yet completely ready (there is no
Profile table created).

Now the profile will be created by two signals:

- user registration, which should take care of most cases
- user login, which should handle remaining corner cases (and was
  present already to support user accounts migration)

This should finally fix issue #102 and issue #132 by removing code which
caused them.