Float "About the data" section left, not right, on PPT.

This looks better on narrower screens, and fixes a bug
in the display on larger screens.

See: https://www.pivotaltracker.com/s/projects/911874/stories/73812576
for details of the bug.
Merge pull request #611 from alphagov/bugfix/page-per-thing-styling

Float "About the data" section left, not right, on PPT