Improve accessibility of modal dialogs (#974)

* improve details modal accessibility

* improve tag modal accessibility

* fix overlays in archive and shared pages

* update tests

* use buttons for closing dialogs

* replace description list

* hide preview image from screen readers

* update tests

bookmarks/styles/theme/modals.css

@@ -78,7 +78,7 @@
       margin: 0;
     }
 
-    & button.close {
+    & .close {
       background: none;
       border: none;
       padding: 0;