Differentiate `variant_id` flavors

[grosscol]

Issue or current state

variant_id is a variable name that has different meanings depending on the context.
These are easily confused and not entirely obvious which meaning is expected even when they appear in context.

• In sequences it's expected to be the VCF format of chrom-pos-ref-alt
• In the browser routes, e.g. /variant/api/snv/<string:variant_id> it is expected to be a dbSNP rsID

Resolved when

Different expectations of content have different variable names.

• var_ref_id for the variant reference identifiers.
• var_vcf_id for the VCF style variant identifiers.