Improve API endpoint deprecation

[chriskapp]

Currently a user can change the status of each endpoint, whether it is in Development, Production, Deprecated or Closed. A user has not the option to define a specific date when the endpoint should move into a specific state. So we should add an option that the user can provide a date at which the endpoint automatically becomes i.e. deprecated or closed. Then we could also adopt the deprecation header s. https://tools.ietf.org/html/draft-ietf-httpapi-deprecation-header-01 This relates also to #297