[Vue Rewrite] Fetch Item Bugfix + other cleanup

[devlinjunker]

Related to #748

Follows #2339

🗒️ Summary

Fixed Bug in retrieving items from backend

• Was due to using the end of the items array in the route components as the offset parameter to the backend
• Stored reference to the last id number retrieved when making backend feed item calls, and use this for offset parameter instead

Moved all FeedItem... components into feed-display/ directory

Moved all backend calls to ...service.ts files in dataservices/ directory

• Added basic unit tests for these

✅ Checklist

• Code is properly formatted
• Sign-off message is added to all commits
  - [ ] Changelog entry added for all important changes. (Can we add the skip-changelog label to prevent the check for changelog? )

📷 Visual

All Items loading:

➡️ Up Next

• Reuse VirtualScroll component for:
  • display items in a folder
  • display all items in database
• Sidebar Actions (WIP)
• use ?selected=<id> url query parameter to store and parse which Feed Item is displayed in FeedItemDisplay
• Quick access to Settings
• Update Documentation
• Mobile friendly layout
• Warning and Error Messages
• Audio/Video Feeds