Updates the message editing process to do a local 'echo'.
On slow connections, now there is visual confirmation of the edit,
similar to when sending messages. The contains_backend_only_syntax
logic and check are the same as there.
We showing "(SAVING)" until the edit is completed, and on successful
edit, the word "(EDITED)" appears. There's likely useful future work
to do on making the animation experience nicer.
Substantially rewritten by tabbott to better handle corner cases and
communicate more clearly about what's happening.
Fixes: #3530.