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/comments/proposals_spec.rb

@@ -35,7 +35,7 @@ describe "Commenting proposals" do
     expect(page).to have_link "Go back to #{proposal.title}", href: proposal_path(proposal)
 
     within ".comment", text: "Parent" do
-      expect(page).to have_selector(".comment", count: 2)
+      expect(page).to have_css ".comment", count: 2
     end
   end
 
@@ -247,7 +247,7 @@ describe "Commenting proposals" do
       expect(page).to have_content "It will be done next week."
     end
 
-    expect(page).not_to have_selector("#js-comment-form-comment_#{comment.id}")
+    expect(page).not_to have_css "#js-comment-form-comment_#{comment.id}"
   end
 
   scenario "Reply update parent comment responses count" do
@@ -363,7 +363,7 @@ describe "Commenting proposals" do
         expect(page).to have_css "img.moderator-avatar"
       end
 
-      expect(page).not_to have_selector("#js-comment-form-comment_#{comment.id}")
+      expect(page).not_to have_css "#js-comment-form-comment_#{comment.id}"
     end
 
     scenario "can not comment as an administrator" do
@@ -419,7 +419,7 @@ describe "Commenting proposals" do
         expect(page).to have_css "img.admin-avatar"
       end
 
-      expect(page).not_to have_selector("#js-comment-form-comment_#{comment.id}")
+      expect(page).not_to have_css "#js-comment-form-comment_#{comment.id}"
     end
 
     scenario "can not comment as a moderator", :admin do