1. Use a submodule for dir-index-html. This isn't a go dependency and treating
   it like one is more trouble than it's worth. We don't actually need to
   checkout the submodule unless we need to regenerate the assets.
2. Avoid a runtime dependency on dir-index-html (may shave a few bytes off the
   final binary?).
3. Remove unused code.
4. Avoid bundling unused files.
5. Switch to a maintained version of go-bindata.
6. Use go mod to manage go-bindata.