Final tweaks to actually validate a URL upon modifying a bookmark, last bits to iron out custom icon/thumbnail corner cases, and moved updateCustomIcon function out of bookmark.js file(since it doesn't actually use the chrome bookmark API) and put it into newtab.js.

Note: if you add "file://*/*" to the permission entry in manifest.json, you could use images on-disk for the custom thumbnails. But that would be your call if you'd like to change it.