[#817] Add two sets of breakpoint variables instead of just one

Option takes in a dimension in em or px. By default we go off 3x the
sidebar width to switch to medium mode (one-column vs multiple columns),
and 4x the sidebar width to switch to large mode (keeps the multiple
columns; whitespace difference mostly)
[#817] Use medium / large media query in Tabula Rasa and Funky Circles

Also adjust whitespace in Funky Circles to reduce padding around the
page on medium-sized screens. (as a proof of concept of the division
between medium/large)
[#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
Merge pull request #820 from afuna/multiple-breakpoints-bug-817

A working base for per-style changes as per #817