Labs flag for default open/close right panel behaviour

[daniellekirkwood]

As discussed in this issue we will implement a labs flag that allows the users to choose the default behaviour for the right panel in Element.

• When the labs flag is ON the right panel remains open, regardless of switching rooms.
• Rooms have the panel open on the last view of that panel in that room
• Rooms where the panel was closed, or is being opened for the first time, will default to the People list
• Labs flag copy: "Right panel stays open (defaults to room member list)" and then putting that in the "Rooms" section of Labs