PM's should not be reported to HipChat

This plugin is sending notifications to HipChat when private messages are sent. (Observed with Discourse v1.4.0.beta2 +2 and version 1.0 of this plugin.)

In the case of new user sign-ups and the "Welcome to {{forum}}!" PM, it's almost useless because only admins can actually click the link to see who the new user is (by way of seeing who the PM was sent to). In all other cases (PMs between users), it's actively creepy to see a feed of even just the titles.