Revert "Replace metalsmith-tagcleaner with a custom marked renderer"

[36degrees]

Reverts #2276

This has introduced some unexpected changes – notably where there is no whitespace between a paragraph and a list, the list items are not made into a list but instead end up part of the paragraph.

For example:

Remember, you must: - tell users about the cookies your service sets on their device - let users accept or reject any cookies that are not essential to providing your service

We think this might have been introduced by the update to Marked, as v4.0.15 includes a change that sounds highly related, but just reinstalling the earlier versions of marked and jstransformer-marked doesn't appear to resolve the issue.

There's a Gist of the differences caused by this change here: https://gist.github.com/36degrees/61bccc9f64a35252f6911c3bf377cac5

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