Extract plugins service

Starting to work on a real `comments` plugin finally brought the opportunity to start the extraction of `plugin` handling.

This has been mentioned in a lot of PR's already. Right now it's really just a one-to-one transport from `MainCtrl`, but already useful, as the plguins can now be injected into other services.

Will grow to something very important soon.

Breaks compatibility with the old main templates - but they aren't really used anyway. The exercise demo is also broken by this, but it needs more work in any event.