refs #13547

* sets headings correctly for better accessibility
* layout.phtml: corrects skip link to search + hides it in advanced search
* footer.phtml: corrects powered by section
* header.phtml: corrects header (accessibility), improves scalability
* corrects adv. search labels and headers (also requires changes to advanced_search.js)
* corrects adv. search x-buttons
* corrects lightboxes/modals accessibility, corrects modal close button
* corrects Breadcrumbs description
* corrects accessibility of search box
* adds modal description, improves modal accessibility (also requires changes to lightbox.js)
* adds missing accessibility-related translations
* adds initial batch of improvements from https://github.com/vufind-org/vufind/compare/master...crhallberg:accessibility-master
* removes Browsealoud Trial Code
* re-arranges header to make more room for search box
* fixes and adapts header for XS, SM, Desktop