Improve login management and session operations

[virtualtam]

Follow-up to #1086

Separation of concerns

• move the IP ban management to a dedicated class
• add get() and set() methods to SessionManager to wrap $_SESSION[...] operations
• add getVisibility() and setVisibility() methods to SessionManager
• move storeLoginInfo() and logout() logic from SessionManager to LoginManager

Security

• use safe hashing functions and comparison operators for all hashes and passwords:
  • password_hash
  • password_verify
  • rfc:timing_attack
  • rfc:deprecate-uniqid
• refactor the logic behind open-shaarli as it bypasses login checks in a non-(obvious|expected) way

Improvements

• add ConfigManager->isConfigured() to check if the user has setup Shaarli
• review the session extension mechanism
• review the stay-signed-in mechanism