Consider making ldap/saml settings endpoints wait for memcached's external auth connection before returning

Based on discussion around POST /settings/saml waiting for the external auth connection to be established here, it would make sense to do this for both SAML and LDAP.

This means that when a request is made to enable SAML/LDAP, it won't return until the external auth connection is established with memcached, after which we can expect that endpoints that require a call to memcached, such as the /docs endpoints, will be able to support saml/ldap users.