EditCalendarActvivity: Switch to compose state and kotlin flows (#320)

* Move composable and create previews

* Create composables using models for clarity

* Create composables using models for clarity

* Move computed properties to new EditCalendarModel

* Move toast with success message

* Move model creation

* Minor adjustments
- Separate concerns into two composables
- move save and delete invocations to composable
- fix compose Preview

* Add documentation

* Name the remaining arguments

* Don't pass view model to child composable screens