PLT-1909 Removed the ability to edit system posts (#3013)

* Removed the ability to edit system posts

* Added increased timeout to SpinnerButton unit tests

i18n/en.json

@@ -935,6 +935,10 @@
     "id": "api.post.update_post.permissions_details.app_error",
     "translation": "Already deleted id={{.PostId}}"
   },
+  {
+    "id": "api.post.update_post.system_message.app_error",
+    "translation": "Unable to update system message"
+  },
   {
     "id": "api.post_get_post_by_id.get.app_error",
     "translation": "Unable to get post"