Rewrite the infrastructure that sends feature flags to annotator code, to
support feature-flagging functionality in guest frames, instead of just the host
frame.

 - Remove `FeaturesService` in the sidebar and move the logic for
   syncing feature flags from the sidebar to other frames into
   `FrameSyncService`.  This is a better place since `FrameSyncService`
   knows when frames connect and thus when to send flags to them. The
   logic for monitoring for flag changes is very simple and doesn't
   needs its own service.

 - Within `FrameSyncService`, send "featureFlagsUpdated" notifications to
   both host frames and guest frames.

 - Convert the functions and global variables in `annotator/features.js`
   into a `FeatureFlags` class, which encapsulates storage of flags,
   access to them and notifying consumers when flags change.

 - Instantiate `FeatureFlags` instances in the host frame and guest
   frame and update the flags stored in them when "featureFlagsUpdated" events
   are received from the sidebar.