Google calendar integration

[rekaehreth]

• Admin should create a new calendar for each group.
• Admin should share each group calendar with the google service account.
  • The google calendar can be shared with the users in the group later
  • User can add group calendars to their own google calendar (invite based on user email)
• Calendar events can be linked to training entities
• Calendar events are automatically synced with db
  • If a group has a calendar
  • Trainings are created based on events created in google calendar
  • Google calendar events are created based on based on trainings created in application