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/scripts/pollfeed.php

@@ -47,7 +47,7 @@ if (!$feedsub instanceof FeedSub) {
 }
 
 if ($feedsub->sub_state != 'nohub') {
-    echo "Feed is a PuSH feed, so we will not poll it.\n";
+    echo "Feed is a WebSub feed, so we will not poll it.\n";
     exit(1);
 }