Tidying up getUser calls to profiles and some events

getUser calls are much more strict, and one place where this was found was
in the (un)subscribe start/end event handlers, which resulted in making the
Subscription class a bit stricter, regarding ::start and ::cancel at least.
Several minor fixes in many files were made due to this.

This does NOT touch the Foreign_link function, which should also have a more
strict getUser call. That is a future project.

plugins/Blog/classes/Blog_entry.php

@@ -158,9 +158,10 @@ class Blog_entry extends Managed_DataObject
         // Use user's preferences for short URLs, if possible
 
         try {
-            $user = $profile->getUser();
-            $shortUrl = File_redirection::makeShort($url,
-                                                    empty($user) ? null : $user);
+            $user = $profile->isLocal()
+                        ? $profile->getUser()
+                        : null;
+            $shortUrl = File_redirection::makeShort($url, $user);
         } catch (Exception $e) {
             // Don't let this stop us.
             $shortUrl = $url;