routes: replace hardcoded routes in models with route helpers.

Add a Routes module that gives models access to route helpers outside of
views, and use it to replace various hardcoded routes.

test/unit/user_feedback_test.rb

@@ -7,7 +7,7 @@ class UserFeedbackTest < ActiveSupport::TestCase
       gold = FactoryBot.create(:gold_user)
       member = FactoryBot.create(:user)
       dmail = <<~EOS.chomp
-        @#{gold.name} created a "positive record":/user_feedbacks?search[user_id]=#{user.id} for your account:
+        @#{gold.name} created a "positive record":/user_feedbacks?search%5Buser_id%5D=#{user.id} for your account:
 
         good job!
       EOS