Use go:embed for serving static UI files

[williamhbaker]

Now that we are using go1.16, we can refactor the static asset server to use go:embed rather than go-bindata.

[williamhbaker]

While working on #21785, I found that since go:embed doesn't do any kind of asset compression, using it as-is to embed the UI resulted in a very large increase in the compiled binary size.

To use go:embed for embedding the UI files, we would need to compress the UI files prior to embedding them, and handle serving the compressed files appropriately. As noted in #21785, something like https://github.com/vearutop/statigz might provide the solution.