Include schedule links in Slack notifications

[Proximyst]

What would you like to see!

When a Slack notification comes and looks like this:

There are time periods that are unassigned in Testing Testing on-call schedule.
From 1 Nov 00:00 to 1 Nov 01:00 (your TZ)

I would love to see the Testing Testing mention link to the schedule, e.g.:

There are time periods that are unassigned in Testing Testing on-call schedule.
From 1 Nov 00:00 to 1 Nov 01:00 (your TZ)

Product Area

Schedules

Anything else to add?

No response

[github-actions]

The current version of Grafana OnCall, at the time this issue was opened, is v1.11.5. If your issue pertains to an older version of Grafana OnCall, please be sure to list it in the PR description. Thank you 😄!