[WIP] Re-implement hiliting annotation cards on hover

... over the highlights in the document.

As discussed in #1728 

Done: hover on the highligts
Todo: hover over the cards
I am aware that I breaking the conventions here; the problem is that the [ngClass directive](https://docs.angularjs.org/api/ng/directive/ngClass) (which [we use](https://github.com/hypothesis/h/pull/1729/files#diff-22eac557e789ec06ce4836e330e8e943R37) for adding the class conditionally) chokes on the original version.