Gitlab LDAP authentication without querying account or anonymous binding

*Note*: This tutorial was last tested with gitlab 8.5 installed from source. This tutorial aims to describe how to modify a Gitlab installation to use the users credentials to authenticate with the LDAP server. By default Gitlab relies on anonymous binding or a special querying user to ask the LDAP server about the existence of a user before authenticating her with her own credentials. For security reasons, however, many administrators disable anonymous binding and forbid the creation of special querying LDAP users.