HTML ticket thread, inline images and draft support

*This is a fundamental change to the message processing engine for osTicket*

This enables (optionally) HTML for the ticket thread, as well as for canned responses and email templates. It also ships with a new set of HTML templates sporting a nice-looking theme.

The ticket thread is now equipped with inline image support. Images sent as inline attachments in emails or uploaded directly via the web interface or API are shown inline in the ticket thread, with optional download links.

The proprietary Redactor editor is used in place of the nicEditor used previously due to better handling of images

**Outstanding**
  * ~~Placeholder image placement for external images~~
  * html2text engine is still buggy
  * PDF printing does not support HTML