Simplify rendering a banner

Now the banner component accepts either a banner or a section and loads
the banner if it's a section, so we don't have to add the `@banners`
variable in several controllers.

spec/components/shared/banner_component_spec.rb

@@ -7,6 +7,70 @@ describe Shared::BannerComponent, type: :component do
     expect(page.find(".banner")).to have_content "Vote now!"
   end
 
+  it "renders given a section" do
+    create(:banner, web_sections: [WebSection.find_by!(name: "debates")], title: "Debate banner")
+    create(:banner, web_sections: [WebSection.find_by!(name: "proposals")], title: "Proposal banner")
+
+    render_inline Shared::BannerComponent.new("debates")
+
+    expect(page.find(".banner")).to have_content "Debate banner"
+    expect(page).not_to have_content "Proposal banner"
+  end
+
+  context "several banners available in the same section" do
+    before do
+      create(:banner,
+             web_sections: [WebSection.find_by!(name: "debates")],
+             title: "First banner",
+             description: "First description",
+             target_url: "/first_target"
+            )
+
+      create(:banner,
+             web_sections: [WebSection.find_by!(name: "debates")],
+             title: "Second banner",
+             description: "Second description",
+             target_url: "/second_target"
+            )
+    end
+
+    it "only renders one banner" do
+      render_inline Shared::BannerComponent.new("debates")
+
+      expect(page).to have_css ".banner", count: 1
+    end
+
+    it "consistently renders one banner" do
+      render_inline Shared::BannerComponent.new("debates")
+
+      if page.has_content?("First banner")
+        expect(page).to have_content "First description"
+        expect(page).to have_css "[href='/first_target']"
+        expect(page).not_to have_content "Second banner"
+      else
+        expect(page).to have_content "Second description"
+        expect(page).to have_css "[href='/second_target']"
+      end
+    end
+
+    it "only renders active banners" do
+      Banner.first.update!(post_ended_at: 1.day.ago)
+
+      render_inline Shared::BannerComponent.new("debates")
+
+      expect(page).to have_content "Second banner"
+      expect(page).not_to have_content "First banner"
+    end
+
+    it "does not render anything with no active banners" do
+      Banner.all.each { |banner| banner.update!(post_ended_at: 1.day.ago) }
+
+      render_inline Shared::BannerComponent.new("debates")
+
+      expect(page).not_to have_css ".banner"
+    end
+  end
+
   it "does not render anything given nil" do
     render_inline Shared::BannerComponent.new(nil)