build(aio): add staging environment

You can now specify what environment you are building
by add it to the `yarn build` command. For example:

```
yarn build -- --env=stage
```

Moreover the `deploy-to-firebase.sh` script will automatically apply the
appropriate environment.

aio/.angular-cli.json

@@ -31,6 +31,7 @@
       "environmentSource": "environments/environment.ts",
       "environments": {
         "dev": "environments/environment.ts",
+        "stage": "environments/environment.stage.ts",
         "prod": "environments/environment.prod.ts"
       }
     }