Add support for dynamic documents

[csillag]

Currently, we can only properly handle static documents. Any document changes remove our ability to react properly.

However, many news portals use dynamic methods to load content, sometimes on scrolling, and to support annotating that content, we need to support dynamic pages.

Fortunately, we have (or at least had) 99% ready code that is smart enough to recognize document changes, and

• [ ] Verify if the existing anchors are still valid
• [ ] Check if the annotations that were previously declared orphan can be anchored now.

These abilities make it possible to support modern news portals, and to use real-time collaboration where both the document contents and the annotations change on the fly.

[csillag]

This depends on #116.

[klemay]

User request via Zendesk: https://hypothesis.zendesk.com/agent/tickets/2422

Originally, the user faced issues with annotations being orphaned because of a problem with the publisher's code. The publisher has since fixed the issue and the user expected that their annotations would be re-anchored.