Show talk button only when JS enabled or talk page not empty

Add hidden class to talk button in PHP if no content on talk page.
Remove hidden in talk.js (only when JS enabled) to hide button for
users, which disabled JS in thier browsers.

Bug: 67819
Change-Id: I4311e23c6796548c91ace799ce66fb6d38242e17
Merge "Show talk button only when JS enabled or talk page not empty"