Mock remote census responses in tests using XML

By using real XML responses developers will be able to understand better
how the integration works (the data flow), and the correspondency between
`remote_census` settings and their place at a real XML response.

As `stubbed_responses` methods were removed from the model layer now the
stubbing part should be managed from the test environment code so also
added a new helper module `RemoteCensusSetup` that can be used anywhere
where we need to call the web service.

Co-Authored-By: Javi Martín <javim@elretirao.net>

spec/system/admin/signature_sheets_spec.rb

@@ -77,6 +77,11 @@ describe "Signature sheets" do
   end
 
   context "Create throught all required_fields_to_verify of custom census api", :remote_census do
+    before do
+      mock_valid_remote_census_response
+      mock_invalid_signature_sheet_remote_census_response
+    end
+
     scenario "Proposal" do
       proposal = create(:proposal)
       visit new_admin_signature_sheet_path