Constrain annotation to transcript in video player

When annotating videos transcripts, the user should only be able to annotate text that is part of the transcript and not text outside of that (such as eg. UI elements). This means that:

- If users select text that is outside of the transcript, they shouldn't see the Annotate / Highlight controls appear.
- Quote selectors (or other selectors) captured on videos shouldn't include any text outside of the transcript

A possible mechanism for this might be for the Hypothesis client to expose a way for the host page to configure the root element used by the `Guest`, and for the guest to ignore selections outside of this element.