Use SVG for own usage of FontAwesome

[silverwind]

We have the FontAwesome icon font in use for a couple of icons and Fomantic-UI ships a custom version of it in their icon font. We can't change the usage in Fomantic-UI but we can replace our own usage with SVG using the files provided in https://www.npmjs.com/package/@fortawesome/fontawesome-free.

Maybe it's even an option to switch to Octicons completely, I think overall it's just 20 or so icons we use from FontAwesome.

Depends on #12157.

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[silverwind]

I consider gradually switching everything to Octicons as the solution here.