generic and optional interface tutorials

[RGreinacher]

Add a description / introduction text (md) and annotated payload to compare to each interface_type - optional. If present, a "Interface tutorial" option is available, once per annotator. The description provides what is to do in the example and how to do it. If all goals are reached (the annotated payload is equal to the target payload), the tutorial is mastered. This is an addition to the existing interface preview / test_payload option.

• new interface_type attributes
  • target_payload
  • tutorial_description
• markdown support for rendering the tutorial_description
• if tutorial details are present
  • "preview" button changes to "tutorial"
  • each annotator has to do the tutorial for each new interface_type they're confronted with once; option to redo it as often as she / he wants.