backend: use Flow in Webpack config (#780)

Summary:
Over the past few commits, I have accidentally removed all Flow errors
from the Webpack config. We can now use Flow on that file to prevent any
new errors from creeping in.

Test Plan:
Running `yarn flow` suffices.

wchargin-branch: backend-flow

config/webpack.config.backend.js

@@ -1,4 +1,4 @@
-// @no-flow
+// @flow
 
 const webpack = require("webpack");
 const ModuleScopePlugin = require("react-dev-utils/ModuleScopePlugin");