Feature cocktail for current Demo

This has all the new fun, integrated.

- Always-on Annotations
- Highlighting Mode (with Social View)
- Notifications

This PR is a union of https://github.com/hypothesis/h/pull/615 and https://github.com/hypothesis/h/pull/614, plus some integrating glue between them.

Demo available here: http://23.21.26.107:8080/