The source code for htmlui is now in github.com/kopia/htmlui
GitHub Actions will drop compiled builds in github.com/kopia/htmluibuild
where they are now used as a go module dependency.
This greatly simplifies the build and improves the security,
because Kopia will be consuming pre-built htmlui.
This also means kopia can now installed with embedded UI using:
`go install github.com/kopia/kopia@latest`
* Replace htmlui_fallback.go with go:embed
* Replace go-bindata generated UI with go:embed
* Update site Go version to 1.16
* Update BUILD.md to reflect workflow with go:embed
This is enabled by `kopia server --ui` and can be viewed in a browser
at http://localhost:51515/
Right now it can only list snapshots and policies (barely).
