Comments: Add a reference link back to the highlighted section

[ibrahima]

The annotation comments/thread messages include a reference to the object that they are annotating, so we can add a link to those comments to point back to that section or annotation. This may not always be applicable for all comments, but when it is available it helps with context. Otherwise it can be hard to decipher what a comment means without that context.

Note that if a comment refers to a deleted document section, there will still be a link that just doesn't go to anything. That seems fine for now.

This relates to #7 where the possibility of including annotation comments was previously discussed.

Screenshot of what it looks like:

https://gist.github.com/ibrahima/7fbe8b5f80baa6197406d960d2ab0f32

When you click on a ref link, it highlights the targetted section.