Improved some two factor methods.

In the future, maybe we can do the same with the `find_by_id` methods that return an Option.

Fixed delete user when 2FA is enabled, implemented delete user for admin panel, and the front-end part for invite user. Secured admin panel behind a configurable token.

Implemented proper logging, with support for file logging, timestamp and syslog (this last one is untested)

Implemented U2F, refactored Two Factor authentication, registering U2F device and authentication should work. Works on Chrome on MacOS with a virtual device.

Improved error messagees, implemented delete ciphers, attachments and account, implemented two factor recovery.

Known missing:
 - import ciphers, create ciphers types other than login and card, update ciphers
 - clear and put device_tokens
 - Equivalent domains
 - Organizations