Room upgrade: new members do not have access to the previous room

From riotX-android created by [bmarty](https://github.com/bmarty): vector-im/riotX-android#470

### Steps to reproduce:
- Join a room which is an upgrade of a previous room
- scroll to the beginning of the timeline
- click on "Click here to see previous message" link in the create event banner

### Observed:
- Toast not implemented: "Open not joined room"

Because the user has not joined the previous room, and preview of not joined room is not supported yet on RiotX.

### Other clients behavior:
- Riot-Web correctly displays the room preview with a button "Join the discussion". Clicking on the button correctly joins the room -> OK
- Riot-iOS goes back to the room list screen and do not open the room preview -> KO
- Riot-Android does not display the banner with "Click here to see previous message" -> KO

No file, test, or entry point is named. Start by reproducing the room-upgrade flow: open the previous-message link from the create-event banner as a user who has not joined the previous room, then trace the unjoined-room handling. Done should provide a usable room preview and a way to join, rather than showing the unimplemented toast.

Written by the indexing model from the issue text.