Downgrade to husky v4

[somewhatabstract]

Summary:

The prepare script added by husky causes yarn to be re-entrant in its installs, which can lead to the same tarball being unpacked more than once at the same time, corrupting itself

See yarnpkg/yarn#6312 and yarnpkg/yarn#2629

Issue: none

Test plan:

yarn test

When we deploy to gcloud, it should deploy.

[codecov]

Codecov Report

Merging #456 (06e028c) into master (2769efc) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #456   +/-   ##
=======================================
  Coverage   99.54%   99.54%           
=======================================
  Files          51       51           
  Lines         664      664           
  Branches      128      128           
=======================================
  Hits          661      661           
  Misses          3        3           

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