Allow FeatureToggleConstraint to run w/o warden

Routing specs and controller specs don't set warden on the request
environment. While we could try to re-implement Warden/Devise logic
here, it's much easier to not rely on the environment to be set.

While I'm usually opposed to changing production code to make testing
easier, it does avoid a lot of complication in this case. And maybe it
would be handy in other circumstances to have a more defensive approach
to checking the logged in user.

app/constraints/feature_toggle_constraint.rb

@@ -12,6 +12,6 @@ class FeatureToggleConstraint
   end
 
   def current_user(request)
-    request.env['warden'].user
+    request.env['warden']&.user
   end
 end

spec/routing/stripe_spec.rb

@@ -3,10 +3,6 @@
 require "spec_helper"
 
 describe "routing for Stripe return URLS", type: :routing do
-  before do
-    allow_any_instance_of(FeatureToggleConstraint).to receive(:current_user) { build(:user) }
-  end
-
   context "checkout return URLs" do
     it "routes /checkout to checkout#edit" do
       expect(get: "checkout").