Use l20n bindings

This PR updates js/l20n.js to the version with HTML bindings from https://github.com/l20n/l20n.min.js/blob/master/l20n.js.

Because the HTML bindings will localize the document when document.l10n is ready, we don't need to call update() when the ACE editor is initialized.

I had to change one line in the bindings, though. The upstream bindings use `node.textContent = entity.value`, but I had to change it to `node.innerHTML = entity.value`, temporarily.  See [bug 866100](https://bugzilla.mozilla.org/show_bug.cgi?id=866100).