FAQ: Support offline storage of faqs

[FelberMartin]

[FelberMartin]

This feature has been deprioritized because the current state of the Android app offers only low offline capabilities overall, and we want to discuss these capabilities and requirements another time. For now, we focus on making the FAQ feature work in online mode.

[FelberMartin]

As I already started working on this ticket here is my intermediate status:

• Branch feature/faq-offline-storage contains already the entities and StorageService definitions
• Further todos:
  • Tests for StorageService
  • Update repo to get data from storage and store remote data in db
  • Add tests for the repo