message view: Fix the wrong time shown for message locally echoed.

This commit updates the _rerender_message to update the message_time
string with the current timestamp on the message rerender.

When we locally echo a message, we store a local timestamp that will
generally not be used as it is replaced by the server time in
echo.process_from_server when we confirm receipt of the message.

echo.process_from_server correctly updates the .timestamp field on
the message and triggers a rerender but that rerender reuses
the message_container object without recomputing the
message_container.timestr due to which wrong older timestr was shown
on the message box.

This commit fix this by calling set_timestr in the rerender code path,
alongside calls to update similar data structures like
this._maybe_format_me_message.

Fixes #17655

static/js/message_list_view.js

@@ -1135,6 +1135,13 @@ export class MessageListView {
         this._maybe_format_me_message(message_container);
         this._add_msg_edited_vars(message_container);
 
+        // The timestr of message_container can be outdated if locally
+        // echoed. When the server sends the new timestamp, though the
+        // timestamp gets updated in echo.js, the timestr does not.
+        // This updates the timestr which will be then be used to
+        // update the message_time during rerender.
+        set_timestr(message_container);
+
         // Make sure the right thing happens if the message was edited to mention us.
         message_container.contains_mention = message_container.msg.mentioned;