Fix down migrations

[galbwe]

From the head migration, run alembic downgrade 2021_05_04.

Output:

INFO  [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO  [alembic.runtime.migration] Will assume transactional DDL.
INFO  [alembic.runtime.migration] Running downgrade dc0ca6997b4d -> 9d857b23b162, Database for CNP
INFO  [alembic.runtime.migration] Running downgrade 9d857b23b162 -> 3d9074e4b65e, add assigned column
Traceback (most recent call last):
  File "/Users/wes/projects/partner-finder/backend/venv/lib/python3.9/site-packages/sqlalchemy/engine/base.py", line 1763, in _execute_context
    self.dialect.do_execute(
  File "/Users/wes/projects/partner-finder/backend/venv/lib/python3.9/site-packages/sqlalchemy/engine/default.py", line 717, in do_execute
    cursor.execute(statement, parameters)
psycopg2.errors.UndefinedColumn: column "assigned" of relation "leads" does not exist
...
sqlalchemy.exc.ProgrammingError: (psycopg2.errors.UndefinedColumn) column "assigned" of relation "leads" does not exist

[SQL: 
        ALTER TABLE leads
        DROP COLUMN assigned;
        ]
(Background on this error at: http://sqlalche.me/e/14/f405)