Fix and simplify subscriber creation/lookup/sync

Problem description

There is a lot of code relating to subscriber creation/lookup/sync that is subtly wrong, confusing or unnecessarily verbose. This is the cause of some bugs and also makes fixing others much more difficult.

Proposed resolution

1. In Subscriber::loadByMail() check for blank mail and return FALSE. Hence calling code does not need to.
2. When looking up a subscriber from a User, always use UID rather than mail.
3. Subscriber::getUser() should check for uid 0 and return NULL.
4. Avoid unnecessary synchronisation if there are no shared fields.
5. Move all synchronising into Subscriber::fillFromAccount(). Always synchronise and always include the full set of fields.
6. Add an optional 'create' flag to Subscriber::loadByMail() and Subscriber::loadByUid(). If the flag is set, when lookup fails the function returns a new subscriber initialised from the supplied uid/mail.

#3031919: Bugs if user has blank email address is mostly solved by 1) and 2).
#2937372: Subscriber postSave method saves/invalidates anonymous user (uid 0) is solved by 3) and further improved by 4).