Fix typo in feature toggle for EVL
Be explicit about whitelisting params[:slug]

Pull out our usage of params[:slug] into a helper and be explicit
about running it through the list of available services (again).

Brakeman was reporting a false positive because of our usage of
params[:slug] in the view.
Merge pull request #175 from alphagov/explicit-whitelist-url-params-55648134

Be explicit about whitelisting URL params