Merge branch 'zj-grape-keys' into 'master'

GrapeDSL for Keys endpoint

No changelog item as it doesn't change behaviour
Cherry-picket from !6330

See merge request !6631

lib/api/keys.rb

@@ -4,10 +4,9 @@ module API
     before { authenticate! }
 
     resource :keys do
-      # Get single ssh key by id. Only available to admin users.
-      #
-      # Example Request:
-      #   GET /keys/:id
+      desc 'Get single ssh key by id. Only available to admin users' do
+        success Entities::SSHKeyWithUser
+      end
       get ":id" do
         authenticated_as_admin!