Update colorbox to 1.4.37

This solves the scrollbar problem raised by the silverstrap 2 update.
Improve external reference handling

Moved all external dependencies on its own template
(templates/Includes/Silverstrap.ss) to have cleaner code.
Inline javascript for autotoc handling has been put inside
js/silverstrap.js.
Remove .ss-buttons, no more required by silverstripe-project