support various and multiple content_types on the display side - still have some work to do on the posting side.
rename 'uri' (and parent_uri) to 'mid' (and parent_mid) since these no longer remotely resemble uri's and are actually message_id's. This change is potentially destabilising because it touches a lot of code and structure. But it has to get done and there's no better time than the present.
Merge https://github.com/friendica/red into zpull