Load Font Awesome CSS from jsDelivr CDN

[iBug]

This is an enhancement or feature.

Summary

Instead of using Font Awesome Kits, load its CSS from jsDelivr CDN.

Why?

✔ Font Awesome official source and NPM package
✔ Always up-to-date with latest FA 5 icon sets
✔ Faster everywhere (jsDelivr is the largest public NPM CDN, also works in China)
✔ Loads fewer bytes ([230 KB original, 184 KB gzipped] vs [1.2 MB original, 457 KB gzipped])
✔ No bothering with async and/or defer. Always loaded asynchronously and displayed as soon as it's ready
✔ Works with ads blocker
✔ Works even with JavaScript disabled

Context

Potentially related: #413

[iBug]

If you dislike dependencies on external services, Font Awesome themselves provides CSS-based usage on

https://use.fontawesome.com/releases/v5.13.1/css/all.css

However I can't find a way to automatically track the latest releases in this way (jsDelivr can).

[mmistakes]

This is why I moved to their newer kit feature so I didn't have to constantly bump the dependency since the version number was tied to URL.

The other consideration was I specifically went with their SVG solution, as that was at the time the new and preferred installed method. Web fonts have sort of fallen out of favor, though I guess that might not hold much water since the project is called "FontAwesome" and still the primary way its used.

My understanding is the JavaScript rewrites the <i> tags into inline SVG which may or may not have benefits. I honestly can't remember.

[iBug]

I don't think they make any visual / practical difference, as even the official FA docs says (bottom of the page):

Reference either the /css/all.css or the /js/all.js, which contains everything you need ...

So for end users both methods of loading should be equivalent.

And yes, the JS version does rewrite the <i> tags, which isn't per-se bad. So what about improving it one step further with this new approach?

[mmistakes]

I didn't think it made any visual differences, it was more of a use SVG or webfonts for icons debate and if one was more performant, accessible, etc. Since the SVGs aren't getting inlined into the source HTML I don't think it matters much, so I'm fine with going back to the webfont/CSS solution for icons.

[iBug]

As a follow-up: I just noticed that FA's official kits also have a "latest" reference, for example:

<link rel="stylesheet" href="https://kit-free.fontawesome.com/releases/latest/css/free.min.css">
<!-- or -->
<script type="text/javascript" href="https://kit-free.fontawesome.com/releases/latest/js/free.min.js">

FA's official CDN is based on StackPath, which may be faster or slower. As far as I am concerned, StackPath is also one of the supporters of jsDelivr, so logically jsDelivr shouldn't be slower or less stable than SP.

[gamue]

This changes broke the option to use a self-hosted font-awesome version which is always a topic when looking into privacy issues and analysing the page quality (e.g with dareboost.com). It's also a feature several users might have used int he past (see #2298). Would be great to have that functionality back without the need to overwrite head.html