Merge branch 'sev/xolonium4' into 'master'

Update Xolonium webfonts

This branch updates the webfonts to [Xolonium 4.0](https://fontlibrary.org/en/font/xolonium).

Note that I have deleted the .svg format, because it is [deprecated](http://caniuse.com/svg-fonts).
In return, I have added the new .woff2 format, which offers better compression.

Also note that I have removed Arial and Helvetica as fallbacks, resulting in:
`font-family:Xolonium,sans-serif;`
I consider this preferable, because it falls back to the default sans-serif font,
which should be the "best" font as determined by the system or the user.
I can change it back if desired.

See merge request !4