Add average SNVs per person to stats page

[rileyhgrant]

To be calculated by production team and added to Stats page after

Related: https://github.com/broadinstitute/gnomad_production/issues/1438

[matren395]

Per Person stats are out:

Exomes: https://docs.google.com/spreadsheets/d/12AI_MMB-USEaGq0iAocwhLRzsn7leUL0sLjGBj5AjNI/edit?gid=855071952#gid=855071952

Genomes: https://docs.google.com/spreadsheets/d/1laHF0g8zXubvLKyXBRP1JYaHXfMMQu99clyH9Jt6XuA/edit?gid=1493364530#gid=1493364530

[matren395]

Mega Summary is that, for Autosomal+Par Exomes, for variants passing variant qc:

gnomAD: 94,247.60 SNVs/person
non-UKB: 107,439.08 SNVs/person
UKB: 84,291.42 SNVs/person

for Autosomal+SNP Exomes, for all variants passing variant qc :
gnomAD: 4,749,800.00 SNVs/person

all stats are for all imputed genetic ancestries