Move stripe secrets to vault.
Move Authorize.net secrets to vault.
Move paypal secret to vault.
Move coinbase_webhook to vault.
Move redditgifts_webhook to vault.
POST_comment: delete unused `is_comment`.
Message.add_props: Delete obsolete incorrect comment.
Message: Don't check to_id when determining if an item is new.
PromoteListingController: return 403 for non-sponsors on special sorts.
Use a MoreRecursion for too-deep child comments.
Use a different inventory factor for default frontpage subreddits.