add_message_id_header_and_parent_id_to_message

[amatsuda]

Here's an AR migration that adds two columns to messages table:

• message_id_header: the message-id SMTP header. Suffixed _header so that the column name doesn't look like an FK to another message
• parent_id: reference to another message, used for drawing the mail thread tree. Will be computed from in-reply-to and references SMTP headers