Automate shifting headers down 2 sizes

Move h1 headers to h3 and so on. Means I can write
markdown documents that can exist independently and
I don't have to remember why a top-level header looks
like '### title here'.

Includes correction of all the places where headers
were being used (some of them at the wrong level too).
Make Baskerville the default typeface - lovely
Use $for$ and $partial$ to clean up post lists

Also means some new content on the home page.