[iOS] Option "Sync Dates to Calendar"

[nizmaylova]

User can sync all deadlines and due dates for current course to his Calendar.

Goal: Replicate previous logic implemented in the production edX app.

Work breakdown summary:

• Sync calendar toggle
• Course dates integration with local calendar
• Turn sync off logic
• PLS logic to update the local calendar
• Update local calendar if manual update is off from course dates

What’s not in scope:

Deep link implementation in calendar events because deep links are not created at the moment.