[EventPage] Adds 'Add to Calendar' integration

[tastelessjolt]

Replicate of https://github.com/wncc/InstiApp/pull/286

• [x] Android
  • [x] Fix manifest permissions
• [x] iOS
  • [x] Fix manifest permissions

[tastelessjolt]

Overall working is fine now.

• [ ] Add error handling when a calendar is deleted
• [ ] Add setting in SettingsPage to clear don't ask again defaults

[VIBR0X]

events are added to calendar by default