Make the left panel look sensible for Read-Only Users

[lampholder]

The details of the new guest experience for Riot are on the project plan: element-hq/riot-meta#59

Currently, the left panel shows text inviting the user to drag rooms around. This isn't very helpful for a user who only has read access to the room they're currently in.

[lukebarnard1]

This also applies for registered users FTR. I swear there was an issue for that...

What about this as a stand-in UI that replaces the 4 "Drag Here" boxes of the apocalypse?:

So if there are no joined rooms, just show a message to encourage the user to start some chats/ join some rooms.

[lampholder]

This is so much more straightforward than the thing I was imagining. Lets do this.

For the record, I was thinking of some changes that could:

• include room/DM creation in the roomlist UX:
  
• in the void left by our not yet having joined any rooms, put a little thingy to prompt people to use that UI to create a room/DM:
  
• replace all of the above with placeholder wigglies for read-only users: