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...

lib/activityhandlerplugin.php

@@ -147,7 +147,7 @@ abstract class ActivityHandlerPlugin extends Plugin
     *
     * This will handle just about all events where an activity
     * object gets saved, whether it is via AtomPub, OStatus
-    * (PuSH and Salmon transports), or ActivityStreams-based
+    * (WebSub and Salmon transports), or ActivityStreams-based
     * backup/restore of account data.
     *
     * You should be able to accept as input the output from an
@@ -193,7 +193,7 @@ abstract class ActivityHandlerPlugin extends Plugin
      *
      * This will be how your specialized notice gets output in
      * Atom feeds and JSON-based ActivityStreams output, including
-     * account backup/restore and OStatus (PuSH and Salmon transports).
+     * account backup/restore and OStatus (WebSub and Salmon transports).
      *
      * You should be able to round-trip data from this format back
      * through $this->saveNoticeFromActivity(). Where applicable, try
@@ -324,7 +324,7 @@ abstract class ActivityHandlerPlugin extends Plugin
     }
 
     /**
-     * Handle a posted object from PuSH
+     * Handle a posted object from WebSub
      *
      * @param Activity        $activity activity to handle
      * @param Profile         $actor Profile for the feed