Database cleanup: Users/Cat links

[DerGernTod]

currently, when running CatImportTask we create new users and linking when creating new cats. since we don't have users anymore: get rid of this creation mechanism, user association within the cat class and existing demo-data regarding user-associated cats.

[DerGernTod]

things to make sure they still work:

• [ ] CatImportTask with imported cats
• [ ] gridfieldstatuschangebutton -> createCat
• [ ] contact fields in admin cat view