gnomAD minor allele frequency display

[jstone-dev]

In search results and tooltips for a single variant, display its overall gnomAD allele frequency.

[jstone-dev]

Adding gnomAD minor allele frequency to the CSV response of the variants/data API endpoint might be an efficient way to expose this to the front end, and it would allow us to use MAFs in other ways not limited to display of a single variant.