Add and apply Capybara/RSpec/HaveSelector rule

This rule was added in rubocop-capybara 2.19.0. We were following it
about 85% of the time.

Now we won't have to check both have_css and have_selector when
searching the code.

spec/system/admin/poll/booth_assigments_spec.rb

@@ -222,10 +222,10 @@ describe "Admin booths assignments", :admin do
           expect(page).to have_content "10"
         end
 
-        expect(page).not_to have_selector "#total_system"
+        expect(page).not_to have_css "#total_system"
       end
 
-      expect(page).not_to have_selector "#recounts_list"
+      expect(page).not_to have_css "#recounts_list"
     end
 
     scenario "Results for a booth assignment" do

spec/system/admin/poll/polls_spec.rb

@@ -327,11 +327,11 @@ describe "Admin polls", :admin do
             expect(page).to have_content("10")
           end
 
-          expect(page).not_to have_selector "#total_system"
+          expect(page).not_to have_css "#total_system"
         end
 
-        expect(page).to have_selector "#poll_booth_assignment_#{booth_assignment.id}_recounts"
-        expect(page).not_to have_selector "#poll_booth_assignment_#{booth_assignment.id}_system"
+        expect(page).to have_css "#poll_booth_assignment_#{booth_assignment.id}_recounts"
+        expect(page).not_to have_css "#poll_booth_assignment_#{booth_assignment.id}_system"
       end
     end
   end

spec/system/admin/poll/questions_spec.rb

@@ -51,8 +51,8 @@ describe "Admin poll questions", :admin do
       click_link "Create question"
 
       expect(page).to have_content("Create question to poll Movies")
-      expect(page).to have_selector("input[id='poll_question_poll_id'][value='#{poll.id}']",
-                                    visible: :hidden)
+      expect(page).to have_css "input[id='poll_question_poll_id'][value='#{poll.id}']",
+                               visible: :hidden
 
       fill_in "Question", with: "Star Wars: Episode IV - A New Hope"
       click_button "Save"