feat(github): show relative dates in pull requests

Pull request lists, timelines, commits and replies show how long ago
something happened ("just now", "8 hours ago", "yesterday", "3 days ago")
and the date after 30 days ("on Oct 12"). Hovering a timeline date shows
the full date and time. With pretty formatting off, the plain date and time
are shown.

src/platform/locales/text/zh.json

@@ -4976,6 +4976,10 @@
   "Merge commit message:": "合并提交消息:",
   "Merge commit message": "合并提交消息",
   "1 minute ago": "1 分钟前",
+  "1 hour ago": "1 小时前",
+  "{n} hours ago": "{n} 小时前",
+  "{n} days ago": "{n} 天前",
+  "yesterday": "昨天",
   "Today": "今天",
   "Yesterday": "昨天",
   "Select pull request base and head to view changes": "选择拉取请求的基础分支和头分支以查看更改",