Allow users with multiple email extensions in one Tatami domain

[rlheritier]

Currently, Tatami uses email extensions to group users. Users with a certain extension can only see their domain.
This feature would allow an admin user to create a domain and invite people to it. Also users would be able to request to join a domain. This domain would have users with different email extensions then.
This feature was requested by Stephane and talked about before. The complexity is that there needs to be an admin screen and thoughts about the workflow / user experience.