Render non coding genes using gene glyph

[shashankbrgowda]

When we have only exon features without CDS then genericChildGlyph is used to render the gene.

[garrettjstevens]

looksLikeGene in packages/jbrowse-plugin-apollo/src/LinearApolloDisplay/stateModel/layouts.ts currently returns true if a feature has children that look like mRNA, which in turn have children that look like CDS and exon. We should remove the requirement that mRNA-like children have CDS-like children, to account for non-coding genes like this one.

Then we'll need to make any modifications needed to the gene glyph to draw just exons with no CDS. It would draw exons as shorter gray boxes (like the UTRs look in the screenshot below), with none of the tall, colored CDS boxes.