Embed frontend assets using "//go:embed"

Go 1.16 introduces a new way to embed static assets in a binary using the //go:embed directive and the packageembed.

We currently use a combination of github.com/jessevdk/go-assets end a fork made by me. Using the native way is safer (embedding is done when go build is invoked, instead of go generate), easier to maintain, and it removes 2 external dependencies, which cannot hurt!