The correct 1-1 avatar is used with rooms in which there are only two users with either "join" or "invite" as their membership (importantly, not "leave" or otherwise).
(This is important when a user moves accounts and re-joins previously left 1-1 chats)
React apparently now checks the properties which are set on DOM elements, and
grumbles noisily about unexpected ones. Update BaseAvatar and RoomAvatar so
that they don't set unrelated properties on the DOM elements.
Object.keys() is O(n) and is wasted because we only care if the keys are 1 or 2.
Use `for .. in` instead and return early if there are >2 keys.
Profiling indicates this cuts wasted time from ~74ms to ~32ms for me (who has
a large number of rooms with large numbers of people in them).
