allow phids other than users (i.e. mailinglists)

Summary:
Via the UI adding a mailinglist for CC works, but via
the API currently it shows:

One or more PHIDs were invalid for ccPHIDS

This removes the user validation check for ccPHIDs.

(I left it in for other things like owner since that seems
still appropriate?)

Test Plan:
used arc locally to add a mailinglist to cc

echo '{"id": 2, "ccPHIDs": ["PHID-MLST-ohduchbv4dfimk7opt3r"]}' | arc call-conduit maniphest.update

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10193