Implement mechanism for dynamic slot configuration

[arbrandes]

Description

We need a mechanism for Apps (or routes, or components) to ask the shell for modifications to slots or widgets - in particular the header and footer - when they're mounted, and to remove them when they're unmounted.

This first immediate use case is allowing the Learner Dashboard App to display the expected header.

Suggested implementation strategy

An initial investigation suggests that using SiteContext to provide a setter to siteConfig would be enough. It would thus be possible for any child component to add slot configuration on mount (via useEffect(), []) with an empty dependency array), and remove it on umount (by returning a cleanup function from useEffect()).

[arbrandes]

It turns out this was already implemented in the form of conditional widgets/active roles... it's just that they weren't working. Fixed by #79, though. We can close this, now.