Fix supporting pages using wrong class

Introduced in 46170bfe2853406a56fcb204e05a38ea942777cf
Fix detailed guide layouts

This reverts commit 7345ddcaaf0d6e81bd53460c0edf072778fd10b0.

The bug originated from 46170bfe2853406a56fcb204e05a38ea942777cf.
An attempted fix was made in 7345ddcaaf0d6e81bd53460c0edf072778fd10b0,
but did not entirely fix the problem. Another attempted fix was made
in 442527650ff3d08c87ff4735006b7f0e499f6dd7 but again did not include
a revert of the fix in 7345ddcaaf0d6e81bd53460c0edf072778fd10b0.

This is the revert to bring everything back to the state before
46170bfe2853406a56fcb204e05a38ea942777cf.

https://www.pivotaltracker.com/s/projects/367813/stories/61656156
Merge pull request #1110 from alphagov/61656156-fix-detailed-guide-style

Fix detailed guide styling

https://www.pivotaltracker.com/s/projects/367813/stories/61656156