message_edit: Add message edit local echo.

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.

docs/subsystems/sending-messages.md

@@ -190,6 +190,15 @@ messages.
   primarily done in `exports.process_from_server` in
   `static/js/echo.js`.
 
+### Local echo in message editing
+
+Zulip also supports local echo in the message editing code path for
+edits to just the content of a message.  The approach is analogous
+(using `markdown.contains_backend_only_syntax`, etc.)), except we
+don't need any of the `local_id` tracking logic, because the message
+already has a permanent message id; as a result, the whole
+implementation was under 150 lines of code.
+
 ## Putting it all together
 
 This section just has a brief review of the sequence of steps all in