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/zh-CN.json

@@ -94,6 +94,8 @@
   "home.column_settings.show_replies": "显示回应嘟文",
   "home.settings": "字段设置",
   "lightbox.close": "关闭",
+  "lightbox.next": "Next",
+  "lightbox.previous": "Previous",
   "loading_indicator.label": "加载中……",
   "media_gallery.toggle_visible": "打开或关上",
   "missing_indicator.label": "找不到内容",