MM-23185 - Markdown image hosted by plugins are not shown if l… (#14185)

* avoid image proxy for local images

* added test for local images

i18n/en.json

@@ -1404,6 +1404,10 @@
     "id": "api.file.write_file_locally.writing.app_error",
     "translation": "Encountered an error writing to local server storage."
   },
+  {
+    "id": "api.image.get.app_error",
+    "translation": "Requested image url cannot be parsed."
+  },
   {
     "id": "api.incoming_webhook.disabled.app_error",
     "translation": "Incoming webhooks have been disabled by the system admin."