- Add `ImportAnnotationsService` service to handle import process for
   annotations and show messages when done.

 - Add `importsPending` store fields to track the number of imports in
   flight.

 - When the `import_annotations` feature flag is enabled, show an Import tab
   in the share dialog.

 - Add `ImportAnnotations` component which implements the UI for the
   Import tab and initiates import when the user clicks "Import"

Part of https://github.com/hypothesis/client/issues/5694