limit imap users and create new mailbox string

This should fix #1902. A very simple yet usable approach. Also will it be possible to use specific url schemes mentioned in the same issue.
For preferences: get the allConfig instance from the server container via getConfig() and use setUserValue(). 

For adding users to groups: get the group manager instance from the server container. Get the group instance via get() and (if result is not null) use addUser() method and pass the uid.