Clean up `messages_new_message()` to use `bp_parse_args()`.

Also re-allows previous functionality where members could respond to threads they are alone in. If someone wants to talk to themselves, who are we to stop them?

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8618 cdf35c40-ae34-48e0-9cc9-0c9da1808c22