Enable flow: sourcecred.js and editor/App.test.js (#239)

They were already correct from a typing perspective, so no other changes
needed.

src/cli/sourcecred.js

@@ -1,4 +1,4 @@
-// @no-flow
+// @flow
 require("@oclif/command")
   .run()
   .catch(require("@oclif/errors/handle"));

src/plugins/artifact/editor/App.test.js

@@ -1,4 +1,4 @@
-// @no-flow
+// @flow
 import React from "react";
 import ReactDOM from "react-dom";
 import App from "./App";