Cache and gradually fill `Explore rooms`

#### Is your suggestion related to a problem? Please describe.
An example is in the `Matrix community` space or any of its subspaces. When you hit explore rooms it take ages for it to load, and when you switch back and forth these have to be fetched again.

#### Describe the solution you'd like.
- When the initial list of rooms/subspaces is queried and received from matrix, first populate a list of dummy rooms
- When checking if the rooms are valid and getting metadata check against a local cache and possibly schedule to renew it in the background
- Gradually update the list of dummy rooms with the proper room details