Adds a `$res` SCSS variable set to the path from the root SCSS file to the `res`
directory.
This is a different base path than previously used in CSS URLs (it goes up 3
directories instead of 2), because Webpack will now be resolving images relative
to the root SCSS file, so the path corresponds to a source tree location,
instead of a path in the build output tree.
Defining this variable has two main goals:
* URLs are a bit easier to read
* The path can be overridden, which is needed for external (riot-web) themes
There are various versions of Nunito in circulation, and some have errors in
their metrics or smaller supported character sets. To ensure all users get the
expected experience, don't allow local copies of Nunito to be used.
Fixesvector-im/riot-web#7959.
