4632: Slack direct message notifications

splatter2

What version are you running?

3.0.2

What's the URL of the page this enhancement relates to, if any?

/account/preferences/#settings

Describe the enhancement and the motivation for it.

The new Slack integration in ReviewBoard 3.x is welcome feature. It would be even better if there was a way that would allow users to receive direct messages on Slack in the notifications settings instead of (or in addition to) email notifications.
This way they users don't have to monitor a channel filled with messages they may not care about.

What operating system are you using? What browser?

N/A

Please provide any additional information below.