RightPanel navigation is unintuitive, especially with threads

[ara4n]

Steps to reproduce

While dogfooding threads I'm finding the RightPanel behaviour very unintuitive. Problems include:

• While I may be interested in the threads on one room, i might be interested in something else (members, widgets etc) in another room. I think the RightPanel should remember the setting for each room, rather than it being a global.
• Pinned Messages lacks a back button on the RightPanel, and doesn't get linked from the RoomInfo panel.
• In terms of IA, It's strange that some of the RightPanel options from the topright buttons take you to a child of the RoomInfo panel specific to that room, but some of them are global in scope (e.g. Notifications). Buttons which are room specific should be grouped together somehow - or perhaps we could even show that they are conceptulally children of the main (i) info icon? We have repeatedly seen that users ignore the buttons on the topright (unless they show badge notifications to remind the user they exist, for things like notifs).

[SimonBrandner]

I also think a big part of this is #17376 when looking into spaces

[ara4n]

ah yup, 100%.