Add E-Mail Fallback

[julien731]

Some users have been in a situation where they were unable to get the OTP from their phone or there was a time sync issue preventing them to login using the OTP.

For those reasons it would be a good idea to have a fallback where an OTP could be sent by e-mail upon user request. The OTP would obviously have a longer lifetime.

The questions that remains though are:

• Should this be enabled by default (I believe it should)
• Should admins be able to toggle this on and off
  • How should this be toggled? Option? Constant?