EDITING: image upload

this adds POSTing to `/resources` but does not rewrite the `<img src="...">` tags upon success.

The next PR will rewrite the URL after the POST using the URL from https://github.com/Connexions/cnx-authoring/pull/18 but any chance it could start with `/resources/` so I don't have to strip the protocol, host, and port when adding it to `<img src="..."/>`?

This is related to (but not dependent upon) https://github.com/Connexions/cnx-authoring/pull/19