Add support for contacting on-call engineering through Slack

[t1agob]

Is your feature request related to a problem? Please describe. The existing PagerDuty card component shows the on-call user(s) email information but email is not an effective communication mechanism when working on an ongoing issue.

Describe the solution you'd like I would like to see a Slack icon next to the on-call user information. By clicking that button it would start a DM on Slack with the user.

Describe alternatives you've considered Currently I see the name of the person, open Slack and start a new DM to that user.