PDF generation

[ejmg]

As of now, the front end generates the PDF form. However:

• this means a user can't pull a copy of what they've previously generated/printed/submitted
• the current method is "not pretty" per @zmon
• caching things is nice

Whether we actually store these PDF's is an issue to discuss, but for now it would be nice to be able to reproducibly give these PDFs back, which means moving this into the back end is probably desirable.