Allow editing of tags in highlights view

This change adds an "edit tag" button for the highlight view page, allowing you to change the tab selection on the highlights page. It mostly reuses the code when highlighting a passage in a document, but I had to add some extra fields to mimic how highlights are read from the db from the different endpoints.

This lays somewhat of a framework for #286, as we now have the start and end character index, so if the user selects a subset of the highlight we can do some math to figure out where that is.