Replace Font Awesome with inline SVG icons

Font Awesome 4 adds 106kB of dependencies on each page and require to
query multiple assets. It also sometimes causes icons to appear after
page loading. Font Awesome 4 is deprecated and replaced by version 5
which is not packaged in every GNU/Linux distributions.

This commit replaces icons with inline SVG which does not require
external assets, does not require an additionnal dependency and is
widely supported by modern browsers. It makes the page loading faster
and enables us to no longer require fonts-font-awesome Debian package.