Improve accessibility (part 7) (#4457)

* fix(media_modal): Keyboard navigation

* fix(column_back_button): Use native button

* fix(media_gallery): Keyboard navigation

* fix(status_content): Make CW content focusable

app/javascript/mastodon/locales/defaultMessages.json

@@ -1113,6 +1113,14 @@
       {
         "defaultMessage": "Close",
         "id": "lightbox.close"
+      },
+      {
+        "defaultMessage": "Previous",
+        "id": "lightbox.previous"
+      },
+      {
+        "defaultMessage": "Next",
+        "id": "lightbox.next"
       }
     ],
     "path": "app/javascript/mastodon/features/ui/components/media_modal.json"