calsub: Add JSON as a supported format for calendar subscriptions

[mastercactapus]

Description:
Adding support for Accept: application/json on calendar subscription requests.

This pull request introduces several changes to the calsub/http.go file, primarily enhancing the functionality to support JSON responses and refactoring existing code to improve maintainability.

• Added a new constant PayloadType and types JSONResponseV1 and JSONShiftV1 to structure JSON responses for calendar subscriptions.
• Modified the ServeICalData function to handle requests with Accept: application/json header, constructing and returning a JSON response with schedule and user shift details.
• Introduced the userNameMap function to encapsulate the logic for fetching and mapping user names from user IDs, replacing the inline code in ServeICalData.
• Refactored the code within ServeICalData to use the new userNameMap function, simplifying the logic for populating user names in the full schedule rendering.
• Added a smoke test that validates both the default response as well as the new JSON response.

Describe any introduced API changes:
Calendar subscription requests including Accept: application/json will return JSON instead of ICAL data.