Honor ticket number in subject line without brackets

We introduced a glitch in 29b37144, which was the patch that introduced
matching on email headers other than the Subject.

I misinterpreted the original `preg_match()` regex, thinking that it was
requiring brackets. Looking at it again today, I see that it didn't. The
current one does, however, require surrounding brackets.

This patch also allows for extra letters or dashes between the `#` and the
ticket number. Eventually we should allow the ticket number itself to have
extra characters -- be formattable.

Fixes osTicket/osTicket-1.8#342