Add favourites subsystem to core

Needed for group messaging and course overview improvements projects, for starring conversations and courses, respectively.

This should be fairly simple, in that we just want to allow components to mark 'items' (essentially {string component, string itemtype, int itemid, int contextid}) as favourite items for a specific user.

This issue just deals with the subsystem. The usage in the respective projects will be issues attached to each epic and will come a bit later.