Use Elasticsearch get requests for variant queries

Currently, queries for individual variants use Elasticsearch search requests. However, variants are stored with document IDs, so get requests can be used instead. This requires implementing [compressed_variant_id](https://github.com/broadinstitute/gnomad-browser/blob/78274d30d4f96a66391601c2e9bf00651dc2f61e/data-pipeline/src/data_pipeline/data_types/variant/variant_id.py#L68-L94) in JS.