Switch to webpack

[Sothatsit]

I think we should switch to using webpack (https://webpack.js.org/) for building the JavaScript (and potentially other aspects) of our codebase. Webpack allows us to just use import statements within the JavaScript, instead of relying on manually entering a list of files to concatenate in compilation.json.

This change poses some problems however:

1. Webpack does not support JavaScript that is not written using modules. All of our JavaScript is not using modules. This means we have to change all of our JavaScript to use modules in order to support webpack... this is a big change. In the long term I do think this change is worthwhile, but it is going to be a difficult change to make.
2. We still need to replace [ver] tags in our JavaScript. This will probably require keeping compile.py, and perhaps making it work with webpack.

I think this change will be worthwhile, but it is a huge undertaking. I'm still not sure of a good way to gradually make this change, as there is no way I can do it all at once.