Update server configurations.

Only an admin can change the SSO settings: the sso object and advanced.sso_redirect_prefix, and, while SAML is enabled, server_hostname, which the SAML service provider URLs are built from. Other users allowed to update the configuration can send those settings back unchanged, as they were read from GET /config, along with the settings they are changing. A request from them that would change an SSO setting is rejected with a 403, and nothing in it is applied.