Webpack configuration

Use Webpack to build the JS files from the client code. With this commit, all the client JS code is built using Webpack.

Resulting file is 1.38MB large in production, and takes about 30 seconds to build+minify on my Macbook Pro.

Left to do:

• Build the CSS files as well as all the other artifacts
• Test it thoroughly. Test on multiple devices. Test with a prefix in the URL.
• More optimizations, such as splitting in chunks.
• Use bootstrap.native (https://github.com/thednp/bootstrap.native) to drop the dependency on jQuery. On hold
• Give it a try to spectrum (http://bgrins.github.io/spectrum/) rc-color-picker colorpicker to drop the dependency on jQuery.
• Build sprites with webpack.
• Remove c3/d3 and keep only dygraphs (or the contrary, but keep only one graph library). On hold

Closes #607 (closed)