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/bookmark-details.css

@@ -36,8 +36,15 @@
     }
   }
 
-  & dl {
-    margin-bottom: 0;
+
+  & .sections section {
+    margin-top: var(--unit-4);
+  }
+
+  & .sections h3 {
+    margin-bottom: var(--unit-2);
+    font-size: var(--font-size);
+    font-weight: bold;
   }
 
   & .assets {

bookmarks/styles/theme/modals.css

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