feat: Add notes for Contacts (#3273)

Fixes #2275

app/views/api/v1/models/_note.json.jbuilder

@@ -2,8 +2,10 @@ json.id resource.id
 json.content resource.content
 json.account_id json.account_id
 json.contact_id json.contact_id
-json.user do
-  json.partial! 'api/v1/models/agent.json.jbuilder', resource: resource.user
+if resource.user.present?
+  json.user do
+    json.partial! 'api/v1/models/agent.json.jbuilder', resource: resource.user
+  end
 end
-json.created_at resource.created_at
-json.updated_at resource.updated_at
+json.created_at resource.created_at.to_i
+json.updated_at resource.updated_at.to_i