This avoids the somewhat confusing visuals of showing messages as
EDITED where the content had not been changed, which also obscured
situations where a message had both been edited and moved.
It's possible we could do better with some sort of fancier block-move
visual styling, but it's a bit tricky to do well given that we support
moving multiple messages at once.
Fixes#20451.
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.
jai2201
Swati Bhageria
Anders Kaseorg
Jack Tiggleman
Anders Kaseorg