message_edit: Don't hide spinner on save success.

Hiding the spinner confusingly flashes the Save button before the
edit view closes. This just prevents that from happening, so that
the sign of success is the rendered, edited message.

web/src/message_edit.js

@@ -1040,7 +1040,13 @@ export function save_message_row_edit($row) {
                 delete message.local_edit_timestamp;
                 currently_echoing_messages.delete(message_id);
             }
-            hide_message_edit_spinner($row);
+            // Ordinarily, in a code path like this, we'd make
+            // a call to `hide_message_edit_spinner()`. But in
+            // this instance, we want to avoid a momentary flash
+            // of the Save button text before the edited message
+            // re-renders. Note that any subsequent editing will
+            // create a fresh Save button, without the spinner
+            // class attached.
         },
         error(xhr) {
             if (msg_list === message_lists.current) {