Calling serverError with a clientError status code (4xx) means it will
automatically default to Internal Server Error (500) which is in the
server error status code range (5xx). That is undesirable.
The portion after StartAtomPubNewActivity would never be reached since
Favorite handles that activity through ActivityHandlerPlugin nowadays.
So I cleaned it up and followed a couple of paths, making stuff prettier.
The code is now more event-driven when it comes to rendering notices
and their related HTML elements, since we can't have direct calls from
core to a plugin.
lib/activitymover.php has a function to move a Favorite activity which
will not happen now. The move must be pluginified and performed as an
event which plugins can catch on to.
Now we have to fix any code in the core which directly uses the Fave class
or any other favorite stuff, since it is pluginised and thus might not be
available on some installations.
