fix up the threading code a little more

- Assign the temp id in 'beforeAnnotationCreated'
- Thread annotations after loading so that they have a `thread`
  property.
- Don't reference threading directly in `setupAnnotation`, instead
  let's leave that to when the reply is created. We need better
  events in Annotator for the Threading plugin to be fully automatic.
use that stretch mixin
reflect reply count in heatmap gradient
fix a heatmap edge case of buckets not merging

I was avoiding expanding buckets beyond the bounds of their
annotations' locations by preventing merging with empty buckets.
However, empty buckets should totally merge with empty buckets.
better colorization of heatmap

Use `interpolateHcl` because that's hella easy.