Compact: add rules page

Our site-wide rules page has been blank when using the compact website
(i.reddit.com).  Oops!
Compact: extend scss compilation docs

This is the only scss file we have, and we don't change it often, so it's not
really integrated into the asset-building pipeline.  In the future, if we don't
get rid of it entirely, we'll probably convert it to less, but in the mean
time, let's make it slightly easier to work with.

I reverse-engineered these instructions, so they might not be exactly what
we've used previously.

In particular, I'm probably using a different version of the gems, which would
explain why the additional import was needed.
Compact: style rulespage a bit

Port over some colors from the desktop site, add a bit of padding so things
aren't right on the borders of the screen, and remove the brick at the bottom
of the page, since it only makes sense with alt-text and mobile clients don't
tend to support hovering.