WebSession: user registration initial Flask port
- Ports user registration view to Flask but moving register view from websession_webinterface to webaccount_blueprint and creating a new WTForm form with validation logic for user registration.
- Fixes issue with vertical aligment of icon legend.
- Fixes issue in webuser when dealing with Flask generated legacy requests instead of proper legacy requests.
- FIXME Unauthorized access page is not ported.
Co-authored-by: Jiri Kuncar <jiri.kuncar@cern.ch>