Follow up to 098ab29d4150a5a39880d59ef6cad5f6795a18fd. Since
we just used a `cattr_reader` on `About` this was not safe
for multisite, since some sites could have the chat plugin
enabled and some may not. Using `DiscoursePluginRegistry` gets
around this issue, and makes it so the chat stats only show
for a site if `chat_enabled` is true.
e195e6f614