[FEATURE] Calendar integration for schedule page

[RiyaMathew-11]

Feature description

Create an Add to Calendar integration for conference sessions in schedule.

This is a special request that came from the community in connection with major conferences like IndiaFOSS:

• An Add to Calendar feature would enable attendees to add those particular talks or sessions to their calendar.
• Enables reminder for attendance.

In addition, a calendar integration for events is also needed.

This is an extension to #224