Improve Slack User Group Notifications and Channel Updates

[mastercactapus]

What problem would you like to solve? Please describe:
Users find the current layout confusing in which they select a channel and then, optionally, update a user group as well. We received feedback from early adopters that this process could be improved.

Describe the solution you'd like:
We would like to clearly separate the intent of updating a user group and creating individual channel notifications by implementing the following changes:

1. Add a dropdown to Schedule on-call notifications, making it similar to contact methods.
2. Decouple Slack user group notification rules from channel notifications. User group notification rules will still include a channel for errors, but will update silently.
3. Keep Slack channel notifications unchanged.

Additional context:
Primary user group issue: #1228

[andrewbenington]

Solved in #3046