Add system console switch for enabling link previews (#5663)

i18n/en.json

@@ -3,6 +3,10 @@
     "id": "April",
     "translation": "April"
   },
+  {
+    "id": "api.post.link_preview_disabled.app_error",
+    "translation": "Link previews have been disabled by the system administrator."
+  },
   {
     "id": "August",
     "translation": "August"