Convert DM to room when a third user is invited

[HarHarLinks]

I had this issue twice this week now (and resolved it manually using /converttodm):

1. start a DM with another user
2. decide to bring a third user into the discussion
3. it's now technically a room except it still appears under people in the room list

If you define the People ("DM") section in the room list to only ever contain the canonical 1:1 DMs, then this is easy to resolve: Upon seeing a join (for the first time via read marker?), check if it's not a DM anymore and move it in the room list.

Related: element-hq/element-web#23175