Add ability to search for highlights in modal

When we have a long list of highlights, scrolling through its entirety in the modal takes up a lot of time, especially if you're focusing on a specific subject. This edit adds a search field in the highlights modal, which bubbles up matching entries to the top. I used re-sorting instead of filtering so we wouldn't affect any existing functionality expecting the

s to be there.

I also chose to persist the value in the search field, since it would be handy to keep the filter around as you're going through your document.