Add vlist builder for more consistent stacking

Summary:
Add a way to automatically build vlists correctly. Previously, we built
vlists manually in ~4 different places, which made it difficult to manage
changes, and led to a large amount of duplication in the less. This also fixes
the vlist construction in safari, where the `display: inline-table` wasn't being
applied because of CSS specificity. This leads to the only significant change in
the huxley tests, with the vertical spacing.

Test Plan:
 - Make sure the tests still work
 - Make sure most of the huxley screenshots didn't change, and that the new
   changes are insignificant.
 - Make sure vlists now work in Safari
 - Make sure the change to the VerticalSpacing screenshot is caused by the
   fix-baseline span now correctly applying `display: inline-table` by creating
   the construct in master and adding `display: inline-table !important` to the
   `.fix-ie` css rule

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D13082