This adds a new `discord_messages` table and extends the persistence
APIs such that `MessagesByID` and `MessageByID` will return user
messages that include their discord message payload.
It also adds APIs to save individual discord messages.
We only write up migrations, down migrations are not always possible in sqlite or
too complex/too expensive. For example to remove a column you would have to duplicate
the table, copy over the data, delete and recreated.
This can be very expensive for some tables (user_messages for example), so should not be attempted.
Notes
One issue we faced multiple times is that updates to user_messages can be very
expensive, leading to slow upgrade times and interrupted migrations. So avoid
writes if not necessary.
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