Instead, only show the box if the draft has been modified since initially
created / updated.

Attachment settings are now exclusively handled by the settings on the
ThreadEntry field on the ticket details form. Enabling attachments as well
as tuning attachment validation settings are all handled from the one place
now.

  * Include a `bk` column to store the storage backend
  * Include a `signature` column which represents a repeatable hash of the
    file contents
  * Rename `hash` to `key` since it isn't a real hash

Also add images on pages to the drop-down list for image choosing, and fix
display of images on the client portal.

Including adding support to the TCL uninitialized variable reader to ignore
class-static variable access as well as detect inline functions and their
closure arguments.

Process inline attachments in thread entry and support inline images in
piped emails

Support inline images across the system, with draft support

Migrate to a single attachment table
    That way we don't need a new table for everything we need to attach an
    inline image to (like a signature, for instance)

Add richtext support for internal notes

Implement images on site pages

* Image paste in Redactor
* Make non-local images optional
* Placeholder for non-local images
* Fix local image download hover
* Don't re-attach inline images