Better handling of areas with many genes

[abought]

Description

If a region contains very many genes, they are not all shown on the genes track. It is possible to drag and expand the genes track, but this is not always very obvious.

This ticket tracks pre-existing items noticed during aggregation test demo.

Steps to reproduce

Scroll to a plot region with a lot of genes. On the newest develop branch, the following index.html query params will work): ?chrom=22&start=21552103&end=22052103

Proposed fixes

Two options:

• Better visual indicator that track can be expanded, or...
• Expand genes track by default

[abought]

Initial triage notes (see sample commit above): the cause has been identified, but the candidate fix (borrowing from intervals track) is not optimal and has some odd side effects.

We'll hold this one until after the Broad meeting for followup.