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/components/sdg/targets/plain_tag_list_component_spec.rb

@@ -55,7 +55,7 @@ describe SDG::Targets::PlainTagListComponent do
     render_inline component
 
     expect(page).to have_css "li", text: "target 1.1"
-    expect(page).to have_selector "a", count: 1
+    expect(page).to have_css "a", count: 1
     expect(page).to have_link "2+",
                               title: "2 more targets",
                               href: "/debates/#{debate.to_param}"

spec/components/sdg/targets/tag_list_component_spec.rb

@@ -60,7 +60,7 @@ describe SDG::Targets::TagListComponent do
 
     render_inline component
 
-    expect(page).to have_selector "a", count: 2
+    expect(page).to have_css "a", count: 2
     expect(page).to have_link "target 1.1"
     expect(page).to have_link "2+",
                               title: "2 more targets",