[entropy panel] within CDS annotations

[jameshadfield]

A future improvement that was discussed as part of #1684 is to allow within-CDS annotations, such as Spike RBD or HA epitope sites. nextstrain/augur#1280 includes a possible JSON schema. f102a11 implements a prototype of this:

• The annotations should only be shown when the CDS is selected to avoid crowding of the lower (nav) CDSs
• A single CDS may have multiple overlapping segmented annotations
• A potential UI is that clicking on the annotation shows a light background to make it clear which positions are within the region. Hovering on bars could also indicate this.