Reorganise the release checklist.

Reorganise the release checklist.

Mostly I'm rearranging things because of the new workflows that git
makes available - it's now possible (and indeed sensible) to prepare a
lot of stuff in a fairly relaxed manner in local checkouts, and then
the process of going live with the release has a lot less manual
writing of stuff and a lot more mechanical 'git push' and running of
update scripts.

However, there's one new item that was actually missed off the
previous checklist: turning off nightly pre-release builds after
making the release they were a pre-release of. Ahem.