Registration: the email binding failed when the application is backgrounded

[giomfo]

The polling mechanism used to wait for the email validation is stopped when the app is backgrounded, and it is not launched again on app resume.

To reproduce:

• start the registration process by providing a username and a password
• submit an email, the app is then waiting for the email validation:

- background the app, and wait for 30 seconds before resuming it. The UI is buggy: there is no loading wheel, and 2 buttons are displayed:

This may introduce an issue if the user validates his email on another terminal while the iOS app is backgrounded. Indeed the account is created without the associated email if he presses the "Valider" button

[pixlwave]

Closed by #6469 and the new authentication flow.