Add `dm_sent` field to infractions database model

[TizzySaurus]

Refers to bot#1864 and site#610.

Make the necessary database model changes to this file in order to support infractions having a dm_sent field in the database which is shown upon !infraction command.

The field will be a nullable boolean (pre-existing infractions will have the value set to null, and future infractions will be set to true/false accordingly).

RELEVANT PR TO BE MERGED WITH THOSE OF bot#1864 AND site#610.

[TizzySaurus]

I'll implement this shortly, along with the other change in regarding the Infractions model.

As discussed on discord, I'm planning to do this myself as part of bot#1864.

[jchristgit]

Reassigned 👍