Require valid external service config json before creating/updating external services.

Created by: nicksnyder

Right now, external service config json is validated on the frontend (which yields green squiggles in the frontend). The existence of validation errors doesn't prevent the form from being submitted however; ideally it would.

Either we need to do validation on the backend (e.g. https://sourcegraph.com/github.com/sourcegraph/sourcegraph/-/blob/pkg/conf/validate.go#L76:11), or, if possible and easier, piggy back on the validation already being done in the client by the monaco editor and prevent form submission there (e.g. does this help? https://github.com/Microsoft/vscode/pull/31045).

cc @tsenart since this relates to external services and @slimsag since this relates to backend validation which is already being done for site config.