v3.7.3: Bugfixes and zc210 interoperation

This release contains changes associated with these GitHub issues:

#316: Don't display back-to-top on small/medium devices.
#386: Use the $template class to locate site_specific_styles.php.
#411: Add the zc210 notifier to the account_history_info page's template.
#417: Submitted keywords to AJAX search must be a 'string'.
#419: Display shipping module's error messages in the shipping-estimator.
#421: "Align" with zc210's implementation for the order_status page's template.
#422: Split CSS/JS loading into separate modules; was in common/html_header.php.
#426: ezpages::page_is_ssl removed in zc210; default always to 'SSL'.
#428: Recognize zc210's mobile-menu settings for EZ-Pages.
#429: Add support for zc210's "Featured Categories" feature.
#432: Correct layout inconsistency between "Notification" and "Manufacturers Info" centerboxes.
#434: Correct background-color bleed-through on various card classes.
#435: Centerbox Background Color applied to containing cards, not the wrapper.