clean up the haskell code

[Noon van der Silk]

the site.hs is a bit messy, because it's copied over from the between-books one

in particular, it needs:

• just a bit of general tidy-up
• perhaps we could "automatically" add the "out-image" image to links that leave the site; i added a function addOutImages for that purpose, which can be implemented similarly to the lqip one.