Embed pkg in Go 1.16

I just came across the embed package in Go 1.16. Seems like this could make it easy to package all templates, pages, and static files into the binary and simplify WriteFreely distribution. We might also eliminate the go-bindata dependency for our included schema files.

Maybe something we should move to?

For cross-reference purposes, a related Phabricator task: T536 (https://phabricator.write.as/T536)