Conpherence: Add a capability to restrict room creation
Summary:
This capability controls which users are allowed to create rooms.
By default anyone is allowed to do so, similar to how it was before.
Closes T16021
Test Plan:
* Change the policy for room creation in Conpherence to Administrators
* Sign-in with a non administrator
* Go to conpherence and see that "Create a Room" is greyed out
* Click on "Create a Room" and see an error message warning about insufficient permissions
Reviewers: O1 Blessed Committers, avivey
Reviewed By: O1 Blessed Committers, avivey
Subscribers: avivey, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16021
Differential Revision: https://we.phorge.it/D26375