[#817] Add back the default padding around content in mobile

Give it space to breathe at the edges of the page
[#817] Add css_divide_length, css_multiply_length

String functions which, given a valid css length, will return that
length multiplied or divided by the given number. Handles detection of
whether this is a valid unit, and appends the unit again once this is
done
[#817] Experimental: shrink the sidebar on medium-sized screens

On small screens we drop the sidebar below the content
On medium size screens, maybe we can shrink the sidebar?
On large screens, display as per usual