Add Distribution model

[jonathan-d-zhang]

up migration

INFO  [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO  [alembic.runtime.migration] Generating static SQL
INFO  [alembic.runtime.migration] Will assume transactional DDL.
BEGIN;
INFO  [alembic.runtime.migration] Running upgrade 587c186d91ee -> a62a93704798, add distributions
-- Running upgrade 587c186d91ee -> a62a93704798
ALTER TABLE scans RENAME files TO distributions;
UPDATE alembic_version SET version_num='a62a93704798' WHERE alembic_version.version_num = '587c186d91ee';
COMMIT;

down migration

INFO  [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO  [alembic.runtime.migration] Generating static SQL
INFO  [alembic.runtime.migration] Will assume transactional DDL.
BEGIN;
INFO  [alembic.runtime.migration] Running downgrade a62a93704798 -> 587c186d91ee, add distributions
-- Running downgrade a62a93704798 -> 587c186d91ee
ALTER TABLE scans RENAME distributions TO files;
UPDATE alembic_version SET version_num='587c186d91ee' WHERE alembic_version.version_num = 'a62a93704798';
COMMIT;