Remove webfontloader and simplify web fonts

[adamzap]

This patch removes the webfontloader JavaScript library in favor of using link tags to load web font assets. webfontloader had its last release in mid-2017, and we were using it in a very simple way.

This patch also makes the following related changes:

• Remove all font assets other than woff, which is widely supported: https://caniuse.com/woff
• Flatten font assets into djangoproject/static/fonts/
• Remove the FiraMono-Medium font variant assets since this variant was not being used
• Update outdated font-related references in the styleguide.html template

@bmispelon This is a particularly exciting PR for me because it drops ~7.7 megabytes from the repository. The diff is huge, but the actual changes are small.

If this gets merged, some nice follow-up changes could be:

1. Update fonts to their latest versions
2. Use .woff2 font assets instead of .woff (it's safe)
3. Self-host the Roboto font since Google open-sourced it? Would this be ok license-wise? Remove Google Fonts #1003 is kind of related.

If there's anything I can do to help test this, please let me know. It looks good to me locally.

[bmispelon]

What can I say, I wish all PRs were like this 💚

20k lines deleted, clear instructions on how to verify the changes, links to reference documentation: this is a pleasure and a breeze to review.

A+, would merge again 💯

[bmispelon]

If this gets merged, some nice follow-up changes could be:

1. Update fonts to their latest versions

2. Use `.woff2` font assets instead of `.woff` ([it's safe](https://caniuse.com/woff2))

3. Self-host the Roboto font since [Google open-sourced it](https://opensource.googleblog.com/2015/05/roboto-googles-signature-font-is-now.html)? Would this be ok license-wise?

Yes, yes, and yes (please) 😁