Check Out button disabled

Added some extra javascript that regularly enables/disabled the
“checkout” button dependent on the cart-size
Added support for page-changing for user details

Added an id-reference in the nab-bar and some additional query at the
bottom of nav.php that scans the nav link id tag
.
Buggy on dynamic user login/logout.
Messes with tab-selection (last tabs stays highlighted and unclickable)