Implement `exhibits_url` to fetch exhibits from an external URL

[krassowski]

To make the configuration easier, GalleryManager.exhibits_url should be available allowing to fetch the exhibits from a JSON or yaml file

[krassowski]

Ability to change examples without redeploying would be really useful for conferences where multiple instructors contribute example repositories asynchronously and the person compiling the list may not necessarily have rights to redeploy.