Adopt Diva's namespaced CSS classes and drop the viewer style-isolation hacks

Diva injects its stylesheet globally, and its class names were generic enough to collide with Bootstrap and Cantus's own styles (`.modal`, `.thumbs`, ...). Two hacks were added during the v7 migration to contain that. Upstream now prefixes every class with `diva-`, so both can go and Cantus can use Diva's stylesheet as shipped.

- Update the vendored Diva tree to v7.4.1 release, which prefixed class names.
- Drop the empty style tag that pre-empts Diva's global stylesheet injection.
- Drop the build-time step that generates a `#diva-wrapper`-scoped copy of Diva's CSS.
- Retarget Cantus's own viewer overrides to the prefixed names.
- Rebuild the committed bundles.