PLAIN APP - server message page

- result of POST and DELETE request for plain app displayed as html
  - status code set
  - message displayed
  - redirection
    - automatic or manual according to time needed for server
      to handle the request (based on parameter in application
      configuration)
      - automatic
        - typically simple tasks like mkdir, mv or rm
        - redirection after a few seconds based on another parameter
          from application configuration
      - manual
        - typically for time consuming tasks like upload of a file(s)
        - link provided to user
- minor refactoring of navigation rendering
  - used one method for rendering links-only navigation