ui/schedules: display override events on schedule calendar

[spencerpauly]

• Identified the issue which this PR solves.
• Read the CONTRIBUTING document.
• Code builds clean without any errors or warnings.
• Added appropriate tests for any new functionality.
• All new and existing tests passed.
• Added comments in the code, where necessary.
• Ran make check to catch common errors. Fixed any that came up.

Description:
Currently, the calendar only shows the resulting shifts from an override. This PR adds an override item to the calendar that shows the details of the override as-well. This lets you edit these details straight from the calendar. You can change the details of the override such as who is affected & for what duration. You can also remove overrides straight from the calendar.

Screenshots:

Image of new override event on calendar.

Tooltip when calendar item is selected:

• The pencil icon takes you to the edit override dialog
• The trash can takes you to the delete override dialog

Describe any introduced user-facing changes:
Users will now be able to see overrides on the schedule calendar, and perform edit and delete operations for current/future overrides via the tooltip.