Use checkIfAvailableMail when requesting to change email
Handle exceptions in registration
Refactor ExtraStorage to allow more backends
Add mongo backend
Merge remote branch 'origin/extrastorages2' into extrastorages

Minor adaptions to our classes

Conflicts:
	config-templates/module_userregistration.php
	lib/ExtraStorage.php
	lib/ExtraStorage/Redis.php
	lib/Registration.php
	www/changeMail.php
	www/newUser.php
Actually add support for MongoDB

Also supports token expiration