Add and apply Capybara/NegationMatcher rubocop rule

This rule was added in rubocop-rspec 2.14.0. We were already following
it most of the time.

.rubocop.yml

@@ -483,6 +483,9 @@ RSpec/Capybara/FeatureMethods:
     - scenario
     - xscenario
 
+RSpec/Capybara/NegationMatcher:
+  Enabled: true
+
 RSpec/Capybara/VisibilityMatcher:
   Enabled: true
 

spec/system/comments/budget_investments_valuation_spec.rb

@@ -273,9 +273,9 @@ describe "Internal valuation comments on Budget::Investments" do
       visit valuation_budget_budget_investment_path(budget, investment)
       expect(page).to have_css(".comment.comment.comment.comment.comment.comment.comment.comment")
 
-      expect(page).to have_no_css(".comment-votes")
+      expect(page).not_to have_css(".comment-votes")
-      expect(page).to have_no_css(".js-flag-actions")
+      expect(page).not_to have_css(".js-flag-actions")
-      expect(page).to have_no_css(".moderation-actions")
+      expect(page).not_to have_css(".moderation-actions")
     end
   end