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>