Allow to use OIDC sub claim as user name instead of the email

[lephilousophe]

When using OIDC providers not based on email addresses but providing one, it may be useful to use the sub claim instead of the email one as the username. If the user changes his email address it can still access its ballots.

This PR adds a new boolean setting use_sub in the oidc configuration stance which, when set to true will use the sub claim instead of the email one as the username.