[SCHEMA] So apparently the Message-ID header is not actually required by Gmail.

This commit includes a migration which should be run using `alembic update head1.
I renamed the `message_id` column to `message_id_header` since we're using
properties other places with *very* similar names (see backrefs).

Ironically it was stored messages from Mailbox that don't include the
Message-ID header as required by RFC-2822. See below...

```
mysql> select message.subject from message where message_id_header IS NULL;
+---------------------------------+
| subject                         |
+---------------------------------+
| Tips for Using Mailbox in Gmail |
+---------------------------------+
1 row in set (3.92 sec)
```

Auditors: spang