Packs are now loaded from views, just like upstream, and are
identified by their filenames. The definition of `theme.yml` has
changed as such:
- `pack_directory` is now required
- `pack` is now unused
- `signed_in_preload` has been introduced
Remove the `fallback` property and do not fallback to using another flavour
when a pack is not available in the selected flavour.
Flavours should define all packs, and should they wish to piggy-back on
another one, they can import that other one's pack explicitly instead.
Claire