Instead of adding the assignee to the end of the message body,
we update the message body where the verb is so that the link
formatting at the end of the message is not broken, for example:
"user_a assigned user_b to [issue #XXX title text is here](link)."
Also updates the issue title in the test fixture so that it tests
that only the first instance of "assigned" or "unassigned" in the
issue title is updated for the assignee text.
Also adds punctuation to the issue title in the test fixture to
test the expected behavior for titles that end in a value from
`string.punctuation`.