Add postgres and redis healthchecks for continuous deployment

https://trello.com/c/uAx6eKAQ/2276-activate-continuous-deployment-for-local-links-manager

config/routes.rb

@@ -1,7 +1,10 @@
 Rails.application.routes.draw do
   root to: "links#index"
 
-  get "/healthcheck", to: proc { [200, {}, %w[OK]] }
+  get "/healthcheck", to: GovukHealthcheck.rack_response(
+    GovukHealthcheck::ActiveRecord,
+    GovukHealthcheck::SidekiqRedis,
+  )
 
   resources "local_authorities", only: %i[index show], param: :local_authority_slug do
     member do

spec/requests/healthcheck_spec.rb

@@ -1,8 +1,21 @@
 RSpec.describe "healthcheck path", type: :request do
-  it "should respond with 'OK'" do
+  before do
     get "/healthcheck"
+  end
+
+  it "returns a 200 HTTP status" do
+    expect(response).to have_http_status(:ok)
+  end
+
+  it "returns postgres connection status" do
+    json = JSON.parse(response.body)
+
+    expect(json["checks"]).to include("database_connectivity")
+  end
 
-    expect(response.status).to eq(200)
-    expect(response.body).to eq("OK")
+  it "returns redis connections status" do
+    expect(data.fetch(:checks)).to include(
+      redis_connectivity: { status: "ok" },
+    )
   end
 end