Upgrade mistune

[galund]

Several substantial changes to the Mistune API, requiring alteration to our custom renderer.

Many of the changes do result in a different rendering, and where I've judged these are probably acceptable I've adjusted the tests, rather than attempt to further customise the markdown handling.

This targets a release candidate of Mistune 3, as it didn't seem worth trying to target version 2 given the newer version's impending availability.

[galund]

"remove-text-newlines" isn't removing all the newlines it could/should at the moment :(