Merge the Pipermail eradication branch. The scrubber is also removed.
* Configuration variable `[mailman]filtered_messages_are_preservable` controls whether messages which have their top-level `Content-Type` filtered out can be preserved in the `bad` queue by list owners. * Configuration section `[scrubber]` removed, as is the scrubber handler. This handler was essentially incompatible with Mailman 3 since it required coordination with Pipermail to store attachments on disk.