When LDAP doesn't supply email address allow user to set it

• Zendesk ticket: https://gitlab.zendesk.com/agent/tickets/9692
• Dev link: None
• GitLab version: 8.0

Expected behavior

When a user logs in via LDAP but their LDAP account does not have an email/mail attribute GitLab generates a temporary one like temp-email-for-oauth-mary@gitlab.localhost. Users should be allowed to change this to a legitimate email address.

Observed behavior

Users that login via LDAP get a read-only email box in their profile.

Next steps

/cc @JobV