Fix DMs being marked as with the current user ("me")
Whilst testing various DM paths, @lukebarnard1 found that there were
many failures to add the room as a DM against the correct user. It
turned out most of the failures seen were because the user chosen
was the current user. If the user accepted an invite it would often
be marked as with themselves because we chose the sender of the
join event as the DM user.
This fix makes the DM room setting process the same for both the
inviting client and the invited client. A RoomState.members
event causes the DM room state to be set in the room, regardless
of whether we are currently `joining` (see previous impl.)
The two cases for setting a DM are:
- this user accepting an invite with is_direct
- this user inviting someone with is_direct
This should handle all cases for setting DM state.
