Frontends core logic

This is part of #1994

Core controller-flow logic of frontend bundles.

It adds FrontendPlug that sets private :frontend key, which is used to determine which frontend is in use later on. All frontend-related requests are directed to FrontendController that essentially proxies to relevant FE controller.

All frontend controllers are expected to export fallback/2 action function that is called whenever the controller doesn't have requested action available.

By default (no pleroma.frontends config available) Pleroma is assumed to be the primary frontend.