Implement "Always-On Annotation" and "Highlighting" modes, with Social View

This implements Always-on [Annotations Mode](https://docs.google.com/document/d/12dqspVLnvrLSBt0-YchjsEPt7F_84C5VeT3cMh7ebjM/edit) (#531) and [Highlighting Mode](https://docs.google.com/document/d/1QSKY_Lc5et4F0pwu0d4AttxIME_E4rjaCHzuwQIgiyc/edit) (#530).

This includes the Social Views support for the Highlighting Mode, too.

Obsoletes https://github.com/hypothesis/h/pull/589