Add form-action CSP directive (#1948)

config/initializers/content_security_policy.rb

@@ -41,6 +41,7 @@ if Rails.env.production?
     p.worker_src      :self, :blob, assets_host
     p.connect_src     :self, :blob, :data, Rails.configuration.x.streaming_api_base_url, *data_hosts
     p.manifest_src    :self, assets_host
+    p.form_action     :self
   end
 end