Most of Susy's mixins have been deprecated, `@include container()`, `@include full()`, `@include span()`, `@include prefix()`, `@include suffix()`, `@include gallery()`, etc.
Fixes#1114
Viewing overflowing sidebar content requires scrolling the entire page which is annoying and causes the reader to lose their place.
Use CSS `calc` to approximate height of the sidebar (`100vh` - height of the masthead) and apply `overflow-y: auto` to add vertical scrollbars when needed.
Fixes#706
