Readd AllowThirdPartyFraming option

We want by default no framing of phpMyAdmin, but still some sites might
need to embed it, so configuration option (with appropriate security
warning) is there.

This basically reverts d7e0bed28443e6cf8f41965cc2f27be19c667fa6.
Improved protection against cross framing

We now include CSS to hide the page and display it conditionally after
checking we're in top frame. This adds extra protection for clients who
do not support X-Frame-Options.

See also http://en.wikipedia.org/wiki/Framekiller and
https://www.owasp.org/index.php/Clickjacking_Defense_Cheat_Sheet