Edited: re-design contentType

For now it uses application/edited but this should uses the current type we edit plain/text + edit or file.

docs.jami.net must also be updated (same for clients). The final design must be discussed first.

changed due date to May 31, 2023

added documentation needed BacklogLow swarm labels

assigned to @sblin

added to epic &49 (closed)

changed epic to &51 (closed)

https://review.jami.net/c/jami-daemon/+/25519

closed

mentioned in commit d9d9d08d