Pin and Unpin Functionality

Added Functionality for Admins to Pin and Unpin rooms using commands (Feature #1150)

Added: 

* Pinned and PinnedPriority column to the ChatRoom entity
* Admin Commands to Pin (`/PinRoom roomname [PinnedPriority]`) and UnPin (`/UnPinRoom roomname`) Rooms
* Icon to indicate that a room is Pinned

Modified:

* Sorting of the chat rooms in the lobby.