feature/httpImprovements

[paulrichter99]

Improve the http implementation and overall data storage of different things:

• [ ] remove events from userDto to prevent to much traffic being send on login / getUser()
• [ ] only get the events of the current month on month switches
• [ ] store old data for every month using some form of caching
  • [ ] only get data from backend if an entry/event has changed
• [ ] improve the storage of calendarEvents using some form of caching