Inline images in email without a 'cid:' indicator

It is the defacto standard to reference an inline image in an email by
referencing the Content-ID header of the referenced attachment in the @src
attribute of the image. For instance, `<imc src="cid:image001.png">` where
the `cid:` scheme of the attribute indicates that the referenced image URL
is a separate content of the email. The image attached to the email would
have an accompanying header like: `Content-Id: <image001.png>`. However,
some mail systems, including a certain fax to email service, do not
correctly indicate the location of the image with the content-id URL.
Instead, the referenced image tag would be `<img src="image001.png">`

This patch addresses the issue by searching the message body for all
references to attached content-id's in all @src attributes with or without
the content-id URL scheme indicator. Previously, such images would not be
displayed inline in the ticket thread.

References:
https://tools.ietf.org/html/rfc2392

include/class.thread.php

@@ -1048,7 +1048,7 @@ Class ThreadEntry {
                 // content-id will be discarded, only the unique hash-code
                 // will be available to retrieve the image later
                 if ($a['cid'] && $a['key']) {
-                    $body = preg_replace('/src=("|\'|\b)cid:'.$a['cid'].'\1/i',
+                    $body = preg_replace('/src=("|\'|\b)(?:cid:)?'.$a['cid'].'\1/i',
                         'src="cid:'.$a['key'].'"', $body);
                 }
             }