features: move feature matchers to their own file

This allows to have the same syntax than native Capybara matchers.

spec/support/feature_matchers.rb

@@ -0,0 +1,9 @@
+module Capybara
+  class Session
+    # Find the description of a procedure on the page
+    # Usage: expect(page).to have_procedure_description(procedure)
+    def has_procedure_description?(procedure)
+      has_content?(procedure.libelle) && has_content?(procedure.description) && has_content?(procedure.service.email)
+    end
+  end
+end