Eventually this should be part of a separate mypy ci build, but right
now it's just a general guideline. Future commits and PRs should be
validated for static typing wherever possible.
For reference, the testing commands used for this commit were:
mypy --ignore-missing-imports --pretty --disallow-untyped-calls app/
mypy --ignore-missing-imports --pretty --disallow-untyped-calls test/
f4a087303d