Collect all env vars in one, easy-to-find place (#1149)

* Collect all env vars in one, easy-to-find place

Past state:
* Default environement variables were stored in both client/package.json
and client/__tests__/e2e/config.js
* Constants that should have been linked--like the cellxgene server port
during testing--were repeated.

With this commit:
* All environment variables are parameterized
* All environment variables are packaged in default env files

* Move npm scripts to client Makefile

* Respond to feedback from @seve and @bkmartinjr

client/configuration/webpack/webpack.config.dev.js

@@ -88,6 +88,9 @@ module.exports = {
     new webpack.NoEmitOnErrorsPlugin(),
     new webpack.DefinePlugin({
       __REACT_DEVTOOLS_GLOBAL_HOOK__: "({ isDisabled: true })"
+    }),
+    new webpack.DefinePlugin({
+      "process.env.CXG_SERVER_PORT": JSON.stringify(process.env.CXG_SERVER_PORT),
     })
   ]
 };