oauthclient: oauth users' email validation

• FIX Marks email address of users creating their account with oauth process as invalid.

• FIX Sends a validation email when users create their account with oauth. (closes #2739)

Signed-off-by: Nicolas Harraudeau <nicolas.harraudeau@cern.ch>
Reviewed-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>