Trying again with the chords as submodules

This way we should have the pdf and the cho aligned. Who cares if github
checks it out...

Maybe we can also do some trick with the file dates to avoid updating
all the pdfs every time.
Generate the pdf directly in the right place

This way the dates of the pdfs used in the website are the ones that
matter for hug-chords' make and they are not all rebuilt (and subtly
changed).
Added 'make touch' to mark the pdfs as up-to-date
Generate the pngs in a separate directory from the pdfs

Use the single songsheets as source, not the book, so that make can
still check what's changed. Only look in the 'hug' dir specifically to
avoid the songbook (but we can move it somewhere else).
Carousel images moved to the pngs dir
Generate the carousel from the pngs dir
html updated
Use the https url to check out the submodule

Not sure, but I think the ssh url requires to be member of the group.
Added a command to update the hug-chords repo