Map current oncall shift to a Slack usergroup.
PAGERDUTY_TOKEN: a read-only token for PagerDutyPAGERDUTY_SCHEDULE_ID: the ID of the schedule to querySLACK_BOT_TOKEN: a bot token from a Slack app, permissions necessary:users:read.emailandusergroups:writeSLACK_USERGROUP_ID: the id of the usergroup in Slack