Highlighting images and media doesn't work yet.

[Roznoshchik]

Currently Lurnby's highlighting only works for text content. Ideally you should be able to highlight images, graphs, and charts.

The existing libraries all seem to rely on making a screenshot by recreating the DOM and I'm not positive if that would work in all use cases such as pdfs and the like.

In any case, would need to do a few things for that.

1. Open up an option to take a screenshot instead of create a highlight in reader mode.
2. Create a new db field for storing image location in the Highlight model.
3. Change how the highlight displays if it's an image or if it's text.

[Roznoshchik]

Implemented this using rangy library and tinymce 🍡