Relax access for Controller_Rest_Comments::post_entity() to match std UI.
Add item argument to Comment::can_comment() to ensure we can view the item, too.
Move Controller_UserProfile::_can_view_profile_pages($user) -->
Identity::can_view_profile($user).  Now REST can use the same check.
Add Controller_Rest_User, and use it for an item's owner and a comment's author.
Merge pull request #406 from shadlaws/fixes_20130613_4

Fixes 20130613 4