Make is_personal like owl_message_is_personal

Before we were hard-coding zephyr messages to be personal if they were
on class message, and were private.  All non-zephyr messages were
personal if and only if they were private.  This disagreed with
owl_message_is_personal, which checked the "personal" filter.  Now,
is_personal also checks the "personal" filter.
Messages sent to 'class messages' are not personal

Add the presumably intended $ at the end.