Replace http-proxy by http-proxy-middleware and upgrade webpack-dev-server to 1.16.1+

[mdutoo]

For now, webpack-dev-server has been downgraded to 1.14.1 to avoid 1. its conf-incompatible migration from http-proxy to http-proxy-middleware ("rewrite" function to "pathRewrite" map...) and 2. migrating said conf as well as the production http-proxy to http-proxy-middleware.

But at some point this should be done... unless directly migrating to webpack-dev-server 2 (for now in beta).

For more details, compare:
https://github.com/webpack/webpack-dev-server/blob/v1.14.1/lib/Server.js
https://github.com/webpack/webpack-dev-server/blob/v1.16.2/lib/Server.js