Repurpose the room directory for group rooms

[Half-Shot]

Related to #140, #117.

I was thinking about how we fix public rooms to not be so...public, while still allowing people to find them if they are permitted to. Obviously, this is basically rooms listed in groups.

So my thought is basically could we somehow extend the room directory to include rooms that in groups that you are in. Ideally, I'd like it so the default listing is rooms that you can see because they happen to be in one or more groups that you are joined to (possibly showing the public room list is better if you aren't joined to any). You could also then further filter it down to rooms by groups in the dropdown.

To be clear, I still want the ability to see locally listed local and remote group rooms but it would be quite nice to make groups more:

• Moderated, if you are part of a community, then there is some assurance that the rooms will be to your taste.
• This should make the room listings less about what server you are using and more about what communities you are part of.
• The community pages are frankly a little hard to find, and having the ability to join via the join room button would be quite sweet.