Heartbeat: require wp-util.js

This gives helpful utility methods such as `wp.template` which we'll use to render an underscores template.
We could copy/paste this method, but wp-utils.js is a tiny file plus there may be benefits to making sure
that Achievements always uses the same underscores rendering method as WordPress itself does (for things
like the media gallery changes).
Heartbeat: tidy up unused code and assign wp.template to achievements.template.

The abstraction helps Achievements remain future-compatible with
WordPress if their wp.template() method changes.

See wp.template() in WordPress' wp-includes/js/wp-util.js.
Make DPA_Default::notifications_heartbeat_response() a static function

Most of the methods in DPA_Default could be static, but I don't want to
change them for backpat with people who may have customised the actions.
PHPDoc tidyup for shortcodes class
"dpa-unlock-notice" shortcode is deprecated
Heartbeat: output the notification JS template for the unlock messages