Convert purge logic to plugins

- Removes uuid as these can only have one instance per template (as
  opposed to image effects, which can be applied multiple times).
- Removes the 'enabled' config setting. Enables is simply determined by
  the plugin being saved to the template or not.

Fix message_example module

- Follow-up to #111

Remove unused classes from use statement

Rename messageType to messageTemplate

- Removes unused 'Xss' class
- Removes outdated, and unused, 'String' class
- Fixes #110

Expose "token replace" as checkbox in the UI