Transcript details panel "Structure" section should support non-coding transcript types

[garrettjstevens]

When displaying e.g. ncRNA of an ncRNA_gene, currently the "Structure" section of the transcript details panel is blank.

This should instead show a list of exons and their coordinated that can be edited. So for example, the below display would not show any UTR or CDS lines, but instead a line for each exon. I think we also still need the splice acceptor and donor columns.