Look up transcripts for gene

[jsstevenson]

Makes transcript/exon lookup easier -- user gets a dropdown of transcript options for a gene

[jsstevenson]

Pop up window for searching for transcripts given gene -- reuse elements from MANE transcript utility if possible

[katiestahl]

note from my 1 on 1 today:
combine the genomic coordinates gene and transcript to one dropdown called "genomic coordinates" and allow a user to enter a gene or transcript. If they enter a gene, we should default to whatever default transcript we want but allow a dropdown for a user to change to a different transcript if they want

change "exon coordinates, transcript" to just "exon coordinates"

[korikuzma]

This was completed by @katiestahl in #273