Allow mulitple login attributes

[fschrempf]

Is your feature request related to a problem? Please describe.

As an admin I want to allow my users to use either their e-mail address or their username to login.

Describe the solution you'd like

I'd like to set LDAP_LOGIN_ATTRIBUTES: "uid,mail" and have lico use that setting so both attributes can be used for login.

Describe alternatives you've considered

Switching to an external IdP, though that's out of scope for small setups.

Additional context

• Allow mulitple login attributes with lico owncloud/ocis#7870
• https://central.owncloud.org/t/ocis-ldap-login-via-e-mail/45994.