use go 1.16 new package io/fs

Unify the interface for reading web statics. Now it could read an
OS directory or get the content on live

One could use

//go:embed static
var webFiles embed.FS

anywhere and config dex server to take the file system by setting

WebConfig{WebFS: webFiles}

Signed-off-by: Rui Yang <ruiya@vmware.com>
Co-authored-by: Aidan Oldershaw <aoldershaw@pivotal.io>