Wordfilter Challenge

Community wordfilters enforced as a posting rule instead of a display filter.

Wordfilter Challenge makes a community's word replacements binding. Publishing clients apply the replacements before signing, so the signed comment and its CID already contain the replacement text, and the community node rejects any publication that still contains a filtered word. It implements the wordfilter/v1 contract, so a client written against that contract keeps working with other implementations.