Previously the spec would use a mix of `string`, `URI`
and `MXC URI` for values of a user's `avatar_url` in
different apis (like `m.room.member`, `m.presence`,
CS profile endpoints).
This clarification updates all occurances to be MXC URIs,
prompted by https URIs `m.room.member#avatar_url` events
in the wild.
Signed-off-by: networkException <git@nwex.de>
