Change mentions of PuSH to WebSub

WebSub is probably finalised before we make a release anyway. Here is
the official spec: https://www.w3.org/TR/websub/

Mostly just comments that have been changed. Some references to PuSH <0.4
are left because they actually refer to PuSH 0.3 and that's not WebSub...

The only actual code change that might affect anything is FeedSub->isPuSH()
but the only official plugin using that call was FeedPoller anyway...

plugins/FeedPoller/lib/feedpollqueuehandler.php

@@ -22,7 +22,7 @@ class FeedPollQueueHandler extends QueueHandler
             return true;
         }
         if (!$feedsub->sub_state == 'nohub') {
-            // We're not supposed to poll this (either it's PuSH or it's unsubscribed)
+            // We're not supposed to poll this (either it's WebSub or it's unsubscribed)
             return true;
         }