Use MxcUri instead of String for media URLs.
Fix wrong MXC URIs in tests.
Remove method parse_mxc no longer useful.
Apply new non-exhaustive types: CrossSigningKey, OneTimeKey and SignedKey.
Apply endpoint name change: send_state_event_for_key to send_state_event
The power level depends on a bunch of stuff, if no power level event
exists the default for a room creator is 100 while for every other user
is 0, thus we need to know the room creator.
The sender controls the content of the membership event, since the
content contains profile data (display names, avatar urls) a sender
might incorrectly change the profile of another member inside the room.
This is allowed in the case where the sender is kicking or inviting the
member, this it will self heal once the member re-joins. Still, to
mitigate this a bit we're storing the profile data when we know that the
member sent out the content on their own.
