Add Offline Support

## Prerequisites
- [x] I have searched open and closed issues for duplicates.

## Feature

It would be awesome to have offline support in the Calendar app. Right now, if the app is started while we do not have any internet connection it does not display any event.

**Describe the solution you'd like**

Show all known events in the Calendar while in offline mode and sync possible modifications as soon as we are back online.

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