Add support for m.room.avatar: refactor avatar URLs
BREAKING CHANGE.
Scope each "getAvatarUrl" to be instance methods on the entity it
relates to (Room and RoomMember respectively). By doing this, we
can actually pull out specific state such as the `m.room.avatar`
event more easily rather than keeping it in the global cesspit
of `MatrixClient`.
This was complicated by `getHttpUriForMxc` and `getIdenticonUri`
which were attached to the HTTP API to pull out the `baseUrl` when
crafting the URL. Pull out this dependency out and explicitly pass
it in when crafting the URL. This is trivial to get from
`MatrixClient.getHomeserverUrl()`.
