Add support for full schedule calendar subscription

[mastercactapus]

What problem would you like to solve? Please describe:
Currently, users can generate a calendar subscription for a single schedule in GoAlert, but it only includes their own shifts. This has led to requests for the ability to see all team members' on-call shifts in the calendar subscription.

Describe the solution you'd like:
Support should be added to allow calendar subscriptions to include all users' shifts on a specified schedule, allowing visibility of the entire on-call schedule in external calendar applications (e.g., Google Calendar).

Describe alternatives you've considered:
An alternative would be to manually create calendar events for all team members' shifts, but this can be time-consuming and error-prone.

Additional context:
This feature request has been prompted by community feedback and similar requests over time.

[mastercactapus]

This would also help view team schedules on mobile devices since the GoAlert UI does not support a calendar view there; users could subscribe using a mobile calendar app to view team schedules from their phones.

[wtc-jeff]

+1 for this feature. We have other departments that would like to see who is on call and a calendar subscription showing the schedule would solve this for us.

[Derekt2]

This would help the CTI team on security so we could see who is on shift easily within outlook from our shared calendar.