Bento: provide boilerplate CSS to avoid CLS

TODO:

- [ ] ~A simple `bento.css` stylesheet deployed to CDN.~
- [ ] ~A `bento.js` script that installs stylesheet and provides some utilities.~
- [ ] ~Reduce stylesheet declaration duplications.~
- [x] Include boilerplate CSS in per-element stylesheets
- [x] Decide whether `amp-inline-gallery` and `amp-inline-gallery-pagination` and such should continue [separate](https://github.com/dvoytenko/amphtml/blob/e117dc48154759fd829774aabdb4a715eb415bef/build-system/compile/bundles.config.extensions.json#L228).
- [x] Include CSS for inlines: amp-date-display, amp-timeago, amp-date-countdown.
- [ ] Figure out if we need to cancel stylesheet install when `` is available, and how.