Shares fixes, refactoring and optimizations

[dartcafe]

This PR turned into a bigger refactoring and bugfix after a review regarding share usage.

• eliminated a lot of db access by
  • caching of user objects i.E. instead of loading for every vote
  • Resolve share users directly from the share's properties in case of external shares
• cleaning up session properties
• Generate random names for anonymous users instead of naming them all Anon
• Adding some more tests
• ignore displayName for public share type and only use label
• Some code simplification for better readability and use case separation