"Burn after reading" option has been moved out of Expiration combo to a
separate checkbox.
Reason is: You can prevent a read-once paste to be available ad vitam
eternam on the net.
(cherry picked from commit 190b278402c086ebc4d1a78aae27d1e2666e3e7a)
Conflicts:
css/zerobin.css
index.php
js/zerobin.js
tpl/page.html