Add emergency banner support

- We remove the odd handling of link_text (where it's blanked
  if the link is blank), because the component can handle this
  already.
- In `static`, the library class handling the banner used a class
  variable to memoize the redis client. Because that's a bit of
  a code smell (and makes testing awkward), we replace that with
  a required config value in Rails.application.config (as mentioned
  in the documentation).
- We use the rails cache to cache the call to redis for one minute,
  which seems like a reasonable compromise between traffic to the
  redis cluster and the banner being available (one minute means
  that we're more responsive than the 5 minute cache, giving us
  a bit of flexibility in cache busting to test when the banner
  comes up).

Audit Trail:
- https://github.com/alphagov/static/blob/bb53325994ebb23476f577f8dbb8bc78d04ceac7/lib/emergency_banner/display.rb
- https://github.com/alphagov/static/blob/bb53325994ebb23476f577f8dbb8bc78d04ceac7/app/views/root/_gem_base.html.erb#L21-L44

app/views/govuk_web_banners/_emergency_banner.html.erb

@@ -0,0 +1,11 @@
+<% emergency_banner = GovukWebBanners::EmergencyBanner.new(redis_client: Rails.application.config.emergency_banner_redis_client) %>
+<% if emergency_banner.active? %>
+  <%= render "govuk_publishing_components/components/emergency_banner", {
+      campaign_class: emergency_banner.campaign_class,
+      heading: emergency_banner.heading,
+      link: emergency_banner.link,
+      link_text: emergency_banner.link_text,
+      short_description: emergency_banner.short_description,
+      homepage: false,
+  } %>
+<% end %>

lib/govuk_web_banners.rb

@@ -1,5 +1,6 @@
 require "govuk_publishing_components"
 
+require "govuk_web_banners/emergency_banner"
 require "govuk_web_banners/engine"
 require "govuk_web_banners/recruitment_banner"
 require "govuk_web_banners/version"

lib/govuk_web_banners/emergency_banner.rb

@@ -0,0 +1,33 @@
+require "govuk_app_config/govuk_error"
+require "redis"
+
+module GovukWebBanners
+  class EmergencyBanner
+    attr_reader :campaign_class, :heading, :short_description, :link, :link_text
+
+    def initialize(redis_client:)
+      content = content_from_redis(redis_client)
+
+      @campaign_class = content[:campaign_class].presence
+      @heading = content[:heading].presence
+      @short_description = content[:short_description].presence
+      @link = content[:link].presence
+      @link_text = content[:link_text].presence
+    end
+
+    def active?
+      [campaign_class, heading].all?
+    end
+
+  private
+
+    def content_from_redis(client)
+      Rails.cache.fetch("#emergency_banner/config", expires_in: 1.minute) do
+        client.hgetall("emergency_banner").try(:symbolize_keys)
+      end
+    rescue StandardError => e
+      GovukError.notify(e, extra: { context: "Emergency Banner Redis" })
+      {}
+    end
+  end
+end

spec/requests/emergency_banner_spec.rb

@@ -0,0 +1,30 @@
+RSpec.describe "Emergency Banners" do
+  context "getting a path with the banner partial" do
+    context "with the emergency banner active" do
+      before do
+        allow_any_instance_of(Redis).to receive(:hgetall).with("emergency_banner").and_return(
+          heading: "Emergency!",
+          campaign_class: "notable-death",
+        )
+      end
+
+      it "shows a banner in the page" do
+        get "/emergency"
+
+        expect(response.body).to include("Emergency!")
+      end
+    end
+
+    context "with the emergency banner inactive" do
+      before do
+        allow_any_instance_of(Redis).to receive(:hgetall).with("emergency_banner").and_return({})
+      end
+
+      it "does not show a banner in the page" do
+        get "/emergency"
+
+        expect(response.body).not_to include("Emergency!")
+      end
+    end
+  end
+end

spec/units/emergency_banner_spec.rb

@@ -0,0 +1,55 @@
+RSpec.describe GovukWebBanners::EmergencyBanner do
+  let(:redis_client) { double(hgetall: {}) }
+  subject(:emergency_banner) { GovukWebBanners::EmergencyBanner.new(redis_client:) }
+
+  describe "caching" do
+    context "with a Rails cache" do
+      it "caches calls to the redis client for one minute" do
+        allow(Rails).to receive(:cache).and_return(ActiveSupport::Cache.lookup_store(:memory_store))
+        Rails.cache.clear
+
+        GovukWebBanners::EmergencyBanner.new(redis_client:)
+        GovukWebBanners::EmergencyBanner.new(redis_client:)
+
+        expect(redis_client).to have_received(:hgetall).once
+
+        travel_to(Time.now + 61.seconds)
+
+        GovukWebBanners::EmergencyBanner.new(redis_client:)
+        expect(redis_client).to have_received(:hgetall).twice
+
+        travel_back
+      end
+    end
+  end
+
+  context "with the emergency banner inactive" do
+    describe "#active?" do
+      it "returns false" do
+        expect(emergency_banner.active?).to be false
+      end
+    end
+  end
+
+  context "with the emergency banner active" do
+    let(:redis_client) { double(hgetall: { heading: "Emergency!", campaign_class: "notable-death" }) }
+
+    describe "#active?" do
+      it "returns true" do
+        expect(emergency_banner.active?).to be true
+      end
+    end
+  end
+
+  context "if the call to Redis fails" do
+    before do
+      allow(redis_client).to receive(:hgetall).with("emergency_banner").and_raise(StandardError)
+    end
+
+    describe "#active?" do
+      it "returns false" do
+        expect(emergency_banner.active?).to be false
+      end
+    end
+  end
+end