add shortcut that opens a comment thread for most recent message in a channel (#6549)

i18n/en.json

@@ -747,6 +747,10 @@
     "id": "api.command_shortcuts.msgs.edit",
     "translation": "UP (in empty input field): Edit your last message in the current channel\n"
   },
+  {
+    "id": "api.command_shortcuts.msgs.reply",
+    "translation": "SHIFT+UP (in empty input field): Reply to the most recent message in the current channel\n"
+  },
   {
     "id": "api.command_shortcuts.msgs.header",
     "translation": "#### Messages\n\n"