Include the 'Report a problem' link with JS

If you have JS disabled, the link never appears on the page instead of
being broken and unclickable.

This pulls in changes from "upstream" :facepalm:

https://github.com/alphagov/static/commit/cbf238c73f77ca9fbac8ee279830cad93983a3b4

https://github.com/alphagov/static/pull/375