Set the width of "more info" for small modules

When a module is only half or a third of the width of the page (cols2
or cols3), its "more info" is set to a third of that width.

This commit adds Sass so that the info on a small module is the size
of the module.
Merge pull request #814 from alphagov/79852326-bugfix-more-info-width

Set the width of "more info" for small modules