modularize front end

[rmnldwg]

While implementing the risk app, I noticed there are a lot of HTML templates and static files from the data viewer that I could reuse. However, with plain Django this isn't super straightforward: If a particular element needs some special CSS or JS file, I could not nicely use {% include ... %} and package the necessary static files with it.

Luckily, there is a package called django-sekizai that tackles exactly this problem. Hence, I can start refactoring the front end.