Super users can be specified via an LDAP group definition

Story: As a Quay administrator I want to be able to specify an LDAP query to determine the list of superusers when Quay uses LDAP as its authentication provider so that I can flexibly add or remove superusers without having to update the Quay configuration / restart Quay.

Acceptance criteria:

• an LDAP search query can be specified in the Quay config bundle to resolve to a LDAP group that holds user accounts that are supposed to be super user privileges
• this new setting should be mutually exclusive with the currently existing configuration direct SUPER_USERS