Intergenic classifications - missing c.HGVS treated as error

Create an intergenic classification, for instance:

NC_000009.12:g.99087902A>C

This is intergenic, so will have no transcript thus no c.HGVS

No c.HGVS is treated as an error, so this causes a bunch of problems:

* Resolution errors are added about missing c.HGVS
* Variant is excluded from export
* No link to allele (even though it has one) on classification page
* Link to resolution tool shouldn't appear - links as "http://localhost:8000/classification/hgvs_resolution_tool?genome_build=GRCh38&hgvs=None" which causes errors when visited