Bug: "medium" and "original" id generators are not sufficiently unique

The "original" and "medium" generators only take account of the message body, date and list id.
This is not sufficient to uniquely identify a message. For example, automated services may send subject-only mails, and may send more than one with the same time.

Since the generated id is used as the database key, this means that only one of the messages will be stored in the database.

Start by locating the "original" and "medium" id generators and the database code that uses their results as keys. Reproduce the subject-only, same-time message case, then verify that distinct messages receive distinct IDs and are both stored.

Written by the indexing model from the issue text.