WebStyle: fix content type for legacy publisher

* SimulatedModPythonRequest would wrongly use the request content type
  in the HTTP response headers, and overwrite any content type set in
  in the response.

Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>
WebStyle: option to disable loading of blueprints

* Add new config option CFG_FLASK_DISABLED_BLUEPRINTS which takes a
  comma-separated list of blueprint modules which should be disabled
  (i.e. not registered with the Flask application).

Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>
WebStyle: fix POST requests to WSGI legacy app

* Fixed issue where POST requests to a WSGI legacy app would result in
  an 405 Method Not Allowed error.

* Added new config option CFG_FLASK_SERVE_STATIC_FILES to enable/disable
  static file serving directly from Flask. This is useful when using the
  Werkzeug standalone development server.

Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>
WebStyle: document Flask request processing

* Short documentation for how Flask request processing happens
  and how the legacy applications is integrated.

Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>