Move the logic for scrolling the content to the highlights associated
with an anchor from `Guest` to the content-type specific integration
(`HTMLIntegration`, `PDFIntegration`). The implementations of this logic
in `HTMLIntegration` and `PDFIntegration` are currently the same as
before, but in future this will allow adapting the scrolling logic to
the content type.

In the case of PDFs the logic will need to allow for the fact that an
annotation may be re-anchored to a different element as the document is
scrolled due to the page containing the annotation changing between an
un-rendered and rendered state.

 - Add `scrollToAnchor` method to `Integration` interface and implement
   it for `HTMLIntegration`, `PDFIntegration`
 - Replace `scrollIntoView` call in guest with a call to the active
   integration's `scrollToAnchor` method