Fixes an issue where API tokens belonging to a deleted user were
not invalidated:
- Adds a trigger to delete rows from the api_key stable when the
column deleted is set to true in the users table.
- Adds a trigger to the api_keys table to ensure that new rows
may not be added where user_id corresponds to a deleted user.
- Adds a migration to delete all API keys from deleted users.
- Adds tests + dbfake implementation for the above.
8fc8559076