Replace icon fonts with SVG

[notramo]

Problem

Icon fonts have many problems.
https://www.youtube.com/watch?v=9xXBYcWgCHA

Possible Solution(s)

FontAwesome, or custom SVG

[mgeier]

Related: #390, #1330.

[uncenter]

This is especially important when the default mdBook template is shipping over 1MB of just icon fonts but only uses 9 (?) icons.