Refactor class extenders

[joepio]

We have a bunch of plugins that extend functionality of Class instances:

• Drive (adds children, could perhaps be removed)
• ChatRoom constructs members / fetches messages
• Invite allows for a redirect and updates usages
• Collection constructs members through a custom query

I'm not happy with how it currently works:

• There is some code duplication in get_resource_extended
• It's not really pluggable, contrary to Endpoints (which can be registered on Store)
• Code is in the lib project, but should probably be in server

Potential solution:

Add ClassExtender struct, register these like Endpoints

A ClassExtender should probably contain:

• An optional get and post function (same as endpoint::handle_get / endpoint::handle_post)
• A class Subject