Simplify aggresively

[repl6669]

Simplification ideas [WIP]

Basic

• rename
  • FlowStep → Step
  • FlowHandler → Handler
  • ClassContract → Class (contract meaning) → alias as ClassContract if needed
• remove user relations from Steps and Nodes (make decoupled and configurable for now)
• decouple Fields (probably out of scope of this package - can be moved to new flow-fields, but it is quite nice that it is UI agnostic)
• decouple Schemaless attributes storage (offer it as one of the first party options)
• decouple Medialibrary (make it configurable as first party data storage option)