Add support to get notification emails only for favorite questions/categories

Table qa_useremailsubscription has changed.
+---------------+------------------+------+-----+-------------------+-----------------------------+
| Field         | Type             | Null | Key | Default           | Extra                       |
+---------------+------------------+------+-----+-------------------+-----------------------------+
| userid        | int(10) unsigned | NO   | PRI | NULL              |                             |
| email         | varchar(80)      | NO   |     | NULL              |                             |
| registered    | timestamp        | NO   |     | CURRENT_TIMESTAMP | on update CURRENT_TIMESTAMP |
| favoritesonly | bit(1)           | NO   |     | b'1'              |                             |
+---------------+------------------+------+-----+-------------------+-----------------------------+

We now use the userid instead of the email as primary key to be
able and find the user's favorites if need so.  We still allow a
user to use a different e-mail than the one in his account but not
multiple e-mails.