matrix: Don't display edited messages as new ones
When a user on another device edits a message we want to replace the old message and display the edited one (indicating that the message was edited on the remote side) other wise the user sees lots of seemingly identical messages.
#729 is related but talks about the local side where a user wants to edit/delete a message.
/cc @francois.techene as this confused several users already (one reference https://matrix.to/#/!zaQzHCRfyTXoMIqcxq:talk.puri.sm/$16830895426536oZzby:talk.puri.sm?via=talk.puri.sm&via=matrix.org&via=matrix.debian.social)