Upgrade to Susy 3 and replace grid mixins with new span and gutter functions

Most of Susy's mixins have been deprecated, `@include container()`, `@include full()`, `@include span()`, `@include prefix()`, `@include suffix()`, `@include gallery()`, etc.

Fixes #1114

docs/_sass/minimal-mistakes/vendor/susy/_susy.scss

@@ -1,4 +1,5 @@
-// Susy
-// ====
+// Susy (Un-Prefixed)
+// ==================
 
-@import 'susy/language/susy';
+@import 'susy-prefix';
+@import 'susy/unprefix';