Getting rid of NoticeListItemAdapter, putting more into ActivityHandlerPlugin
and relying on plugins to handle rendering code of the content. This gives us
a lot more structure and consistency in notice structure and allows activity
plugins to stop rendering certain kinds of notices more easily.
There should also be a property for an ActivityHandlerPlugin class to avoid
rendering notices in the ordinary stream, so we don't have to overload stuff.
ffb9d7ad3f