Turn all "extension" features into content providers

Now that we have an API to provide single components from a feature as an optional extension, we can turn all extension features that exposes a content type into real content providers. This concerns NS Fact, NS Poll and NS Form.

The following needs to be done with each of them:
* Change group to Content provider
* Implement hook_defaultconfig_optionals() for each content provider, and add the reference field to ns article as an optional extension.