OStatus: save file records for enclosures

Also stripping id from foreign HTML messages (could interfere with UI) and disabled failing attachment popup for a.attachment links that don't have a proper id, so you can click through instead of getting an error.

Issues:
* any other links aren't marked and saved
* inconsistent behavior between local and remote attachments (local displays in lightbox, remote doesn't)
* if the enclosure'd object isn't referenced in the content, you won't be offered a link to it in our UI

lib/activity.php

@@ -1044,6 +1044,7 @@ class Activity
     public $id;      // ID of the activity
     public $title;   // title of the activity
     public $categories = array(); // list of AtomCategory objects
+    public $enclosures = array(); // list of enclosure URL references
 
     /**
      * Turns a regular old Atom <entry> into a magical activity
@@ -1140,6 +1141,10 @@ class Activity
                 $this->categories[] = new AtomCategory($catEl);
             }
         }
+
+        foreach (ActivityUtils::getLinks($entry, 'enclosure') as $link) {
+            $this->enclosures[] = $link->getAttribute('href');
+        }
     }
 
     /**