Slack Notifications / Reminders

[Tethik]

Currently only email notifications are supported by Gram. Unfortunately these have a tendency to go ignored. Specifically reviewers will sometimes miss that they had a review assigned to them.

As such we would like to consider using slack messages as this is less likely to be ignored by users.

Further things that could be considered (might be better as separate issues, but good to consider if working on this issue):

• Reminders exist in the Klarna deployment of Gram - something that could be considered for migration into the open source repo.
• Adding bot interactivity - allowing lookups, previews etc via a slack bot.
• User level configuration to allow users to select what kind of messages they want and what channel they want them via.