Look into replacing bbp_encode_bad for bbPress

[johngodley]

Several attempts have been made to work around the content filters in bbPress. As new problems come in they workarounds get more elaborate. It may make more sense to replace bbp_encode_bad (the main cause of these issues) with a version that accepts and sanitises block content, possibly in a similar way to how WP itself does it for posts.

See #123