Implement "Always-On Annotation" and "Highlighting" modes

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).
(Except the planned Social Views support for the Highlighting Mode, which is coming soon.)

Please review.