doesn't work with LDAP

According to Gitlab API documentation `login` is required field (afterwards is used for LDAP)

- login (required) - The login of user
- email (required if login missing) - The email of user
- password (required) - Valid password

This patch works with LDAP authentication. However we might consider using `email` field when `login` is missing.
Please, change  label to "Username or email" like a gitlab login form placeholder