Add button to edit highlight from tag view

Fixes #181

This is a big change, because we previously didn't populate the highlights variable on the tag view. We also didn't store the document_id of each highlight, since they were always the current document.

Another problem is highlight_add commands don't have the content of the highlight. We need that content to update the view on changes (previously the view didn't update for changes, but we need it now that we can make changes). I worked around this by copying the content to the new highlight when getting an update, and by reloading the view entirely if we try to display the tag view and we are missing content.

This needs integration tests.