[webctl] Generate static data directory on upgrade (closes #2167873)
- if the folder already exists, ``upgrade`` asks for deletion,
- add an option (``generate-staticdir``) to allow skipping this task.
- add an option (``staticdir-path``) to specify the static data folder path.
The ``gen-static-datadir`` command allows to specify the target folder but
there is otherwise no way to retrieve this information during upgrade.
