Fixes #387

proof of concept only. for localwiki.

Needed a stop marker after the threshold, so that the area above
the top bucket (under the toolbar) doesn't show as having annotations.

Merging buckets is now done in the initial pass. The entire heatmap
is calculated in two passes. The first creates the buckets while the
second counts the replies.

Use `interpolateHcl` because that's hella easy.

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.

- 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.

fixes #158

After this we should be able to remove some references to threading
and instead use the data in the annotation.

The annotation is the message.

fixes #376

This avoids some of the whitespace seen and reported, such as in issue

This addresses some of #370