Build thanks in Webpack build

Here is an example implementation of a way to feed the future "thanks" page.

I wrote a quick and dirty script to list dependencies and their license, as well as their website. It is embedded in Webpack client build, same way as themes were handled. The current overhead on the compilation is about < 0.1s, so definitely not noticeable.

I embedded it in the client build as it seems to me this is the best point to compute it, when all the dependencies are available. Let me know what you think about it.

Closes #576 (closed).