astakos: Use webproject URLs as urlconf in reverse

Use "synnefo.webproject.urls" as urlconf in the reverse() calls of
`im/models.py/` and `im/user_utils.py`.

The reason why we explicitly use the webproject URLs is because Admin,
which imports functions from Astakos, may use a different ROOT_URLCONF
in order to disable the Astakos/Cyclades API. Therefore, If a user
needed to resend a verification mail from the Admin Dashboard, the
reverse would fail.