Mjulian/sogo login with mail address

Z-push was having problems when trying to sync information from carddav sogo server. Allowing the users to login also with their mail address solves the problem of users using different mail address than their username, ie:
username: mjulian 
mail address: figarcorso@company.lte

To solve this issue, this PR should also be released: Zentyal/Z-Push-contrib#2