Iterate on room directory

Related to https://github.com/vector-im/element-ios/issues/3627

Figma: https://www.figma.com/file/qCxVpd02yzJK26AF5BjsU7/Contextual-FABs-per-tab--BWI?node-id=0%3A1

## What we have so far

- We already have contextual FAB, so I think there is nothing to do on our side for FAB:

- Room directory:

## TODO

- Iterate on Room directory UI (?)
- move "create new room" action to the bottom of the screen
- Current network header with change (currently in the top end menu)
- Join button update
- Change small icon for number of members update
- Remove topic preview (?)