Remove redundant visibility matcher usages

By default, Capybara only finds visible elements, so adding the
`visible: true` option is usually redundant.

We were using it sometimes to make it an obvious contrast with another
test using `visible: false`. However, from the user's perspective, we
don't care whether the element has been removed from the DOM or has been
hidden, so we can just test that the visible selector can't be found.

Besides, using `visible: false` means the test will also pass if the
element is present and visible. However, we want the test to fail if the
element is visible. That's why a couple of JavaScript-dependant tests
were passing even when JavaScript was disabled.

spec/support/common_actions/votes.rb

@@ -5,9 +5,11 @@ module Votes
   end
 
   def expect_message_you_need_to_sign_in_to_vote_comments
-    expect(page).to have_content "You must sign in or sign up to vote"
-    expect(page).to have_selector(".participation-allowed", visible: false)
-    expect(page).to have_selector(".participation-not-allowed", visible: true)
+    within(".participation-not-allowed") do
+      expect(page).to have_content "You must sign in or sign up to vote"
+    end
+
+    expect(page).not_to have_selector(".participation-allowed")
   end
 
   def expect_message_to_many_anonymous_votes