Use react-scripts proxy for development

Setting the `defaultDevConfig.url` to use the default port and
proxying requests via the react-scripts proxy overcomes CORS issues
and reduces friction for ui contributions

ui/package.json

@@ -3,6 +3,7 @@
   "version": "0.2.0",
   "private": true,
   "homepage": ".",
+  "proxy": "http://localhost:80",
   "dependencies": {
     "@material-ui/core": "^4.4.3",
     "@material-ui/icons": "^4.4.3",

ui/src/index.tsx

@@ -17,7 +17,7 @@ import {PluginStore} from './plugin/PluginStore';
 import {registerReactions} from './reactions';
 
 const defaultDevConfig = {
-    url: 'http://localhost:80/',
+    url: 'http://localhost:3000/',
 };
 
 const {port, hostname, protocol, pathname} = window.location;