Replace refute + positive capabara matcher in cucumber steps

In general it's a bad idea to use patterns like:
'refute page.has_content? "something"'.

With most of Capybara's matchers, it'll wait around a bit and try
again if the matcher returns false. Capybara does this to support
Javascripty / AJAXy stuff, but it can cause unnecessary delays (regardless of
driver).

Capybara provides negative versions of all it's matchers, so something like
'assert page.has_no_content? "something"' is the right way to do stuff.

This commit changes 27 instances of the negative pattern in cucumber steps,
and that small change shaves nearly 15 secs off the test run suite.
Merge pull request #979 from alphagov/replace-bad-refute-with-capabara-matchers

Replace refute + positive capabara matcher in cucumber steps