mustache_magic: Provide a magical {{#message}} variable for displaying messages.

Problem/Motivation

It would be great to have a magical {{#mesage}} variable for displaying text as a message.

Proposed resolution

{{#message.ok}}/{{#message.success}}/{{#message.warning}}/{{#message.error}}

• Implement this variable as higher-order section lambda.
• Consider using Drupal's JS message library for displaying messages on the client side.

Remaining tasks

Implement.