search: Hide search providers which are blacklisted by parental controls

search: Hide search providers which are blacklisted by parental controls

If a search provider is installed by an app which is blacklisted for the
current user by their parental controls, don’t show it or results for
it.

Currently, this only filters ‘remote’ (not built-in to the shell) search
providers. This seems fine for now; in future it could be expanded to
also filter built-in search providers, if any of them end up needing to
be filtered.

No corresponding changes need to be made `remoteSearch.js`, because the
results of `loadRemoteSearchProviders()` are filtered in `search.js`.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/465