Change new wgUserRegistration format, put in User:

* This is much more useful if it can be passed to new Date.  As a side
effect, this means straight arithmetic comparisons can be done.
* Add a method for this to mediawiki.user (getRegistrationDate).
* Improve docs on server User::getRegistration method by documenting possibility that data is null.

Change-Id: Id7ae0faa930433876939b73d47fc294975e14fb1
Merge "Change new wgUserRegistration format, put in User:"