make /join accept room ids, for power users without url bars

[ara4n]

[t3chguy]

should it also accept a server_name if you want to join a room over federation then?

[turt2live]

related: #3248
also related: #2925

we could probably use the same approach from #2925 to make it work correctly. An MVP would be just to hope/assume the user is already in the room and they just want to go there.

[lampholder]

I would dearly love for this to be addressed in a way that also solved #3248 so that I can direct my electron client to visit matrix.to links :)

[turt2live]

(for #3248 I was thinking of a /goto <url> command. /join could probably also accept a matrix.to URL with all the server information introduced in the solution for #2925 and magically work)

[turt2live]

Fixed by matrix-org/matrix-react-sdk#2272