add “add to calendar” integration

[ruhilmansi]

feature:
provide one click calendar actions:

• google calendar
• outlook
• apple / ical (.ics download)

example UX:
buttons near event info:

• add to google calendar
• download .ics

[denshaw-09]

@ruhilmansi raise the pr

[ruhilmansi]

@ruhilmansi raise the pr

event page?