Certificate Templates - allow configuration external asset dependencies

[sei-bstein]

Currently, certificate templates "specify" external dependencies like images and fonts by simply declaring them in the HTML template. While this works, there are two issues:

1. It'd really be better to standardize loading the cert's background image so that it displays nicely on an A4 sheet.
2. External dependencies like images and fonts are not explicitly declared, so Gameboard doesn't actually know about them. Thus, it can't export these assets when exporting a game or whatever.

Allow declaration of external asset dependencies (then update import/export to export them along with the cert).