Remove dependency on fonts.googleapis.com

[hannesbraun]

Is your feature request related to a problem? Please describe.

The app is currently depending on fonts.googleapis.com. This is caused by some of the stylesheets of the themes:

cookbook/static/themes/darkly.min.css
cookbook/static/themes/flatly.min.css
cookbook/static/themes/superhero.min.css

Describe the solution you'd like

I think removing this dependency and providing a vendored version of the font makes more sense. The two reasons for this are:

• increased privacy
• still being able to access the page on your local network without any issues in case of an internet outage

I was affected by the latter one and had to block fonts.googleapis.com in my browser so that the site would continue to load.

Describe alternatives you've considered

An alternative would be to just refrain from using themes that rely on fonts.googleapis.com. But I think that's rather a workaround than an actual solution.

Additional context

Related: #75