LDAP: add preventSignin
Add configuration of ldap's prevent_ldap_sign_in to these charts.
Provide global.appConfig.ldap.preventSignin which will result in the
templating of the above value.
Documentation update include.
Closes #1675 (closed)
Implements gitlab-org/omnibus-gitlab!3692 (merged)
Relates to gitlab-org/gitlab#15626 (closed)
cc @jamedjo