Try to use browser storage instead of fingerprint in StayConnected (#2881)

Design:

• registration:
  • server sends a proposed TOTP secret
  • if browser has a local storage, it stores the key and use it to generate TOTP
  • else it downloads fingerprint2.js and does like before
  • if server gets a valid TOTP, it stores the TOTP secret in permanent session, esle it does like before
• authentication
  • when cookie exists, server sends the same HTML/JS than registration
  • if browser has TOTP secret in its storage, it uses it
  • else it downloads fingerprint2.js and does like before
  • if server gets a valid TOTP or a valid fingerprint (if no secret were stored) => OK