main: embed assets from www to allows PWD to run standalone (#425)

Generate these assets in config because this package is imported by
most.

Add internal/addgenheader as a simple helper because go-bindata doesn't
output the write header.

Also update test CI workflow to regenerate and check commit is clean.

internal/addgenheader/addgenheader.go

@@ -0,0 +1,26 @@
+// addgenheader is a simple program that adds a DO NOT EDIT style
+// comment at the top of a file. Because some generators do not do
+// this, e.g. go-bindata
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"strings"
+)
+
+func main() {
+	var buf bytes.Buffer
+	fmt.Fprintf(&buf, "// %v DO NOT EDIT\n", strings.TrimSpace(os.Args[2]))
+	fmt.Fprintf(&buf, "\n")
+	byts, err := ioutil.ReadFile(os.Args[1])
+	if err != nil {
+		panic(err)
+	}
+	fmt.Fprintf(&buf, "%s", byts)
+	if err := ioutil.WriteFile(os.Args[1], buf.Bytes(), 0666); err != nil {
+		panic(err)
+	}
+}