Map current oncall shift to a Slack usergroup.

Required Env Vars

• PAGERDUTY_TOKEN: a read-only token for PagerDuty
• PAGERDUTY_SCHEDULE_ID: the ID of the schedule to query
• SLACK_BOT_TOKEN: a bot token from a Slack app, permissions necessary: users:read.email and usergroups:write
• SLACK_USERGROUP_ID: the id of the usergroup in Slack