Fix feedback link + tests (bug 870719)

This fixes up the tests for the feedback modals - investigating the tests found that clicks for the modal weren't happening if the viewport was too small. Also matchMedia doesn't seem to work correctly for the test environment so there's a need to prop that for the click hookup.

Currently it's necessary to teardown the viewport size change. Would be really nice to have a setUp/tearDown available or maybe explicitly setting the viewport should happen each time somewhere in smokealarm/casper.js as an alternative?