Room directory

[bbhtt]

Is your feature request related to a problem? Please describe.

There is no way to browse the list of published rooms in Hydrogen. This is useful when one doesn't know the exact room alias but wants to join.

Additionally when support for joining was added in https://github.com/vector-im/hydrogen-web/pull/358 the room directory feature wasn't included although the corresponding issue mentioned it as being desirable.

Describe the solution you'd like

Hydrogen should:

1. Allow browsing the list of published rooms of one's own homeserver.
2. Allow adding other homeservers to see the list of published rooms there.
3. Allow joining rooms from the room directory.

Describe alternatives you've considered

None

Additional context

None