Fix editing messages by adding or removing leading /me.

Previously, this did not correctly rerender the message to be (or not
to be) rendered as a /me style message.

Fixes: zulip#835.

static/js/message_edit.js

@@ -47,6 +47,7 @@ exports.save = function (row, from_topic_edited_only) {
 
     if (new_content !== message.raw_content && !from_topic_edited_only) {
         request.content = new_content;
+        message.is_me_message = new_content.lastIndexOf('/me', 0) === 0;
         changed = true;
     }
     if (!changed) {

zerver/lib/actions.py

@@ -2334,6 +2334,9 @@ def update_flag(um, should_set, flag):
 
         update_flag(um, wildcard, UserMessage.flags.wildcard_mentioned)
 
+        is_me_message = getattr(message, 'is_me_message', False)
+        update_flag(um, is_me_message, UserMessage.flags.is_me_message)
+
     for um in changed_ums:
         um.save(update_fields=['flags'])