At some point during the riot redesign, the bit of css that lowered the opacity for offline/unavailable contacts
was removed. This makes it impossible to discern presence for a contact unless you hover your cursor over their
avatar.
It's very handy to be able to see presence at a glance without any hovering, so this PR reintroduces the lowered opacity.
I've also slightly decreased the opacity level from 0.66 to 0.5 to make it slightly more noticable.
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
