Add rendermime plugin

Compare PR jupyter/notebook#6885

Author: jitseniesen

spyder_notebook/server/app/package.json

@@ -171,6 +171,7 @@
           "@jupyter-notebook/application-extension:menu-spacer",
           "@jupyter-notebook/application-extension:pages",
           "@jupyter-notebook/application-extension:paths",
+          "@jupyter-notebook/application-extension:rendermime",
           "@jupyter-notebook/application-extension:shell",
           "@jupyter-notebook/application-extension:splash",
           "@jupyter-notebook/application-extension:status",