Add UI to select any alternative transcript

[eweitz]

This lets you see all transcripts for a gene, one at a time. It also shows spliced or unspliced transcript length.

Previously, only the canonical transcript was shown. Genes usually have multiple transcripts, and the alternative transcripts can be important. Now, building on data in #321 (though only using gzip compression for now), these can be seen directly in Ideogram. This lets users learn at a glance how many other transcripts exist for their gene of interest, as well as their gene structure.

Also previously, only the gene's length was shown. "Gene length" is slightly abstract, as a gene is essentially a conceptual container for one or more immature transcripts. "Transcript length" is a more specific kind of gene length, and more directly corresponds to the length of the molecules that get measured in, say, modern gene expression assays. Gene length is still shown in the default tooltip, but transcript length is now shown upon hovering over the gene structure diagram.

This also refines the positioning and display logic for data on strand and biotype.

Here's how it looks:

https://user-images.githubusercontent.com/1334561/202910948-4f210ff6-b5e8-4a12-a023-95da53de63a0.mov

[coveralls]

Coverage decreased (-0.8%) to 85.444% when pulling 190fd6dbf8c1f5164144b5217d282006b44390e3 on all-human-transcripts-ui into 71a277e52aceee833aa7b9b898d8b6887212833f on master.