When revising a fusion variant, allow for optional exon coordinates

[acoffman]

Previously, the revise fusion variant form shared the same validation rules as the "create fusion variant" form. This meant requiring 5' or 3' coordinates to be entered (depending on the properties of the Feature).

However, when revising a representative Fusion variant that may not have coordinates specified, this would render a user unable to provide an alias or additional SO terms.

This changes the rules so that:

• Exons are required if the corresponding transcript is filled in
• Transcripts are required if the corresponding exon is filled in
• Ensembl version and Reference build are required if either transcript is filled in
• Offset and Offset direction continue to work as before

[susannasiebert]

+1