Overlapping annotations for human campaigns + custom JS highlighting library

[kasnerz]

This pull request enables human annotators to create overlapping annotations 🎉

To make it possible, we switched from the YPet.js library to our custom SpanAnnotator implemented in static/js/span-annotator.js.

Internally, collecting annotations is now much more transparent and tweakable. We now also use our SpanAnnotator both for the annotation interface and for the browse interface.

Users can now control whether they want to enable overlapping annotations in the campaign config. (The option is disabled by default.)

This pull request also brings visual improvements 🌈

• We restrict the choice of span colors to a subset of darker, saturated colors using Huebee.js

• We use underline + bold text + text color to denote annotations. (This works well with overlapping annotations!)

demo.mp4

If this pull request is merged, we should:

• update wiki to reflect these changes
• make a new release

Resolves #31