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.

tools.go

@@ -0,0 +1,7 @@
+// +build tools
+
+package tools
+
+import (
+	_ "github.com/jteeuwen/go-bindata"
+)