Context:
In order to track whether a particular user has seen an onboarding feature e.g. the "What's New"-Dialogue we need an API to store this information per user.

The idea is to have a generic api where we can store this state based on a particular key per user.

AT:

• a new secured endpoint  GET /api/onboarding/:key is available that returns a boolean specific to the calling user
• a new secured endpoint  PUT /api/onboarding/:key is available that allows to store a seen flag
• allowed keys are configured, invalid keys rejected with a 404
  • the only key needed now would be whatsnew
• the endpoints are documented