Retry on slack failures

[KatieMSB]

Describe the Bug:
When updating User Groups for on-call schedule notifications, if Slack returns an error, the following message is posted in the schedule's on-call notification Slack channel:

Hey everyone! I couldn't update @<user_group> because I ran into a problem. Maybe touch base with the GoAlert admin(s) to see if they can help? I'm sorry for the inconvenience!

Here's the ID I left with the error in my logs so they can find it:
SlackUGErrorID=<service_id>

This behavior occurs when encountering a fatal_error response from the Slack API. We should implement retry logic in this scenario, or at least offer the user an option to manually trigger a retry.

Expected Behavior:
GoAlert should either automatically retry the request when encountering a fatal_error response from Slack or provide users with the option to trigger a retry within the error message.

Application Version:
All versions as of v0.33.0