Modularize (and clean up) JavaScript

[Lukas Pühringer]

All non-third-party front-end code, except for some scripts that are directly embedded in html templates, is currently implemented main.js.

As pointed out by @SantiagoTorres #14 (comment) and in the header block of main.js we could move out some functionality to separate files.

For instance, it might make sense to split functionality by pages, because e.g. not all event listeners need to be registered on every page.