Add support for gnomAD GRCh38 annotations

[themisteryang]

• This PR adds support for gnomAD GRCh38 annotations in addition to the current support for gnomAD GRCh37 annotations.
• Going forward, all gnomAD annotations will be organized into separate collections in MongoDB, more specifically:
  • GRCh37ExomeAnnotations: for GRCh37 exome annotations;
  • GRCh37GenomeAnnotations: for GRCh37 genome annotations; and
  • GRCh38GenomeAnnotations_chr<chr>: a separate collection for each of the GRCh38 genome annotation chromosomes.
  • At this time, all gnomAD annotations has been imported into the production MongoDB instance and indexed for use on staging and production.
  • The legacy annotations collection will be retained until the changes from this PR have also been merged into production, after which it can be dropped.

Closes #269

[themisteryang]

@jennyziyi-xu The OSMP server workflow diagram (osmp_2.png - #296) will eventually need to be updated to denote support for gnomAD GRCh38 annotations.

[jennyziyi-xu]

@jennyziyi-xu The OSMP server workflow diagram (osmp_2.png - #296) will eventually need to be updated to denote support for gnomAD GRCh38 annotations.

Ok that's a good point.

[themisteryang]

Looks good to me! I wasn't able to test locally with our mongodb instance, so we can iron it out in staging.

If you'd like to test the gnomAD GRCh38 annotations locally, you can try a gene from the X chromosome (e.g., PIGA)! The annotations have been uploaded and indexed for it on our local/test MongoDB instance. Chromosomes 1-8 have also been uploaded (but they haven't been indexed yet).