link to prose.io from website, by doing some fancy jinja magic

The mechanism I used for the "fancy" jinja stuff is really just sloppy and what I did on my last tarbell project when I didn't know any other way, but it should be improved. 

Instead of denoting a zero-based index at the top of the file that matches up with the ordering of the spreadsheet, it would be better to get a reference to the correct spreadsheet row by referencing its id. I need to either find (in jinja docs) or otherwise create a simple little thing that does that lookup.