MM-31713 Server/API: GetUserThread method (#16659)

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

i18n/en.json

@@ -5626,6 +5626,10 @@
     "id": "app.user.get_threads_for_user.app_error",
     "translation": "Unable to get user threads"
   },
+  {
+    "id": "app.user.get_threads_for_user.not_found",
+    "translation": "User thread doesn't exist or is not followed"
+  },
   {
     "id": "app.user.get_total_users_count.app_error",
     "translation": "We could not count the users."