Move state related to received-but-not-yet-applied annotation updates
("pending updates") and deletions from the `streamer` service to a new
`real-time-updates` module in the store.

This will make it possible for React-based UI to reflect this state and
update when it changes. It also removes some usage of Angular-specific
logic (`$rootScope`) from the streamer service.

Applying pending updates still requires a call to the
`applyPendingUpdates` method of the streamer because that currently
needs to trigger side effects and dispatches Angular events.

 - Move `pendingUpdates` and `pendingDeletions` local variables in
   streamer to `real-time-updates` store module

 - Replace calls to `store.{countPendingUpdates, hasPendingDeletion}`
   with calls to the store instead

 - Call store/streamer methods related to real-time updates directly
   from `<top-bar>` instead of passing it down from `<hypothesis-app>`.

   The `<top-bar>` component is not likely to be used outside the app,
   so there is no benefit to the indirection