Move style/script adding from _drupal_theme_initialize() to page rendering

Follow-up from #2160589: Remove drupal_add_js() from theme.inc → .

We currently add scripts and styles in _drupal_theme_initialize(). This part of the logic could be moved to a different function, and done as part of page rendering - only for the responses that needs scripts/styles added. We could either do this as a final step before rendering css/js, or attach the scripts to the $page array. Either way it'd make a full bootstrap lighter.