email: Always include a message-id token

Previously, if the quoted response mechanism was disabled, then outgoing
mail would also not include the message-id token. This breaks the
correlation of email to ticket-thread. Now, the message-id token is always
embedded in HTML emails even if the quoted-response removal system is
disabled.

(Turns out that the message-id token was always included in text bodies.)
Merge pull request #1197 from greezybacon/issue/no-qr-mid-token

email: Always include a message-id token

Reviewed-By: Peter Rotich <peter@osticket.com>