Remove all external dependencies to make it truly single-file & offline

[HarryR]

This is great, but it's annoying to rely on external online dependencies.

It looks like it should be fairly straightforward but tedious to remove jQuery as a dependency, this can be done on a case by case basis to transition away without breaking the app, until finally it's all removed. Many of these could be moved into a small collection of utility functions, such as addClass(el, className). Alternatively native WebComponents could also be used and could similarly simplify the code.

Secondly, while you may really like the Barlow font, most people will be happy with system fonts, for example GitHub uses -apple-system, BlinkMacSystemFont, "Segoe UI", "Noto Sans", Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji".