Only append path to survey URL once

If showSurveyBar is called more than once, the query string gets
appended multiple times.
Add method to set user satisfaction survey URL

In whitehall we want to add different surveys for some pages.
It could be possible to make this a slimmer processor so anything
could set an arbitrary user satisfaction survey, but the survey
is displayed using JavaScript anyway, so configuring it using
GOVUK.userSatisfaction seems like a neater solution.
Merge pull request #345 from alphagov/add-set-survey-url-to-user-satisfaction-survey

Add method to set user satisfaction survey URL