Convert cohort_variant and result primary key types to bigint

Backwards migration is NOT supported.
MySQL, Oracle, and SQLite are NOT supported.
ONLY forward migration in PostgreSQL is supported.
Resolves #222

Signed-off-by: Ryan O'Hara <oharar@email.chop.edu>
Merge pull request #289 from ryanjohara/issue-222

Convert cohort_variant and result primary key types to bigint