Exons are incorrectly numbered - UTRs should not be treated as separate exon

[tonydisera]

For the MANE SELECT transcript for MTHFR, there should be 12 exons, but instead, when hovering over the exons in the variants card, the tooltip shows 14 exons. We need to change the numbering so that the exon number is incremented for every exon. In cases where an exon encapsulates a UTR and a CDS, these both will have the same exon number. In cases where the UTR is separate from the first (or last) exon, the UTR will have its own exon number.