Also check for email address on userExists (next to Avatar and Quota)

This is a one-liner! – I missed it in PR https://github.com/owncloud/core/pull/7931

It also makes sure that the email address is up to date, even if the user has not logged in for a long time (e.g. Remember-Cookie is set). This ensures that the email address is correct anytime when a file is shared with email notification enabled, for instance.

Please review @Xenopathic @karlitschek @leo-b or others

How can you test it?
1. Have LDAP configured and the email attribute set
2. Have a user with an email address. The user should have been logged in once.
3. Change the email address in LDAP.
4. Wait until LDAP cache is invalidated (default 10 min)
5. Look in the sharing dialog for the user
6. Confirm that the email address has been changed in ownCloud to the new value.