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

classes/File.php

@@ -286,5 +286,12 @@ class File extends Memcached_DataObject
         }
         return $enclosure;
     }
+
+    // quick back-compat hack, since there's still code using this
+    function isEnclosure()
+    {
+        $enclosure = $this->getEnclosure();
+        return !empty($enclosure);
+    }
 }