Look into replacing bbp_encode_bad for bbPress

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

Start by reviewing the bbp_encode_bad call path and the discussion in #123. Compare its handling of block content with how WordPress sanitises post content, then determine the scope and acceptance criteria for a replacement that avoids the current bbPress filter workarounds.

Written by the indexing model from the issue text.