slack: allow saving user group ID to schedule notification rules

[mastercactapus]

Description:
This PR adds the slack-ug feature flag, starting support for Slack user group updates for schedule changes.

Which issue(s) this PR fixes:
Part of #1228

Out of Scope:

• non-functional (this PR is data only)
• UI bits are not yet available
• API for listing groups is not available

Describe any introduced API changes:

• new slack-ug experimental flag
• new slackUserGroup target/notification channel type

Additional Info:
Start with make start EXPERIMENTAL=slack-ug

• you must have a slack workspace with user groups enabled (paid only feature)

Testing can be done with the following mutation:

mutation ($input: SetScheduleOnCallNotificationRulesInput!) {
  setScheduleOnCallNotificationRules(input: $input)
}

And variables:

{
  "input": {
    "scheduleID": "<schedule id goes here>",
    "rules": [
      {
        "target": {
          "type": "slackUserGroup",
          "id": "<user group ID goes here>:<fallback channel id goes here>"
        }
      }
    ]
  }
}

You can get user group IDs from this page; you will need your bot token, with the usergroups.list scope:
https://api.slack.com/methods/usergroups.list/test

Channel IDs can be found in the slack app by opening the channel info and going to the bottom.