Hide attachment links for inline images
Merge pull request #758 from greezybacon/feature/thread-inline-images

Reviewed-By: Peter Rotich <peter@osticket.com>
Implement thread `format` field

Thread entries now have a flexible `format` specification, which allows the
system to save the format alongside the original text. This allows the
system not to have to convert everything to HTML when storing in the
database
thread: Implement format for Thread::setBody()
Honor thread body format in PDF export
Use strongly typed ThreadBody instances for output
Hide inline attachments on PDF output