Re-added "po_to_json" in jh's patch

What does this MR do and why?

What

Related issue: #2906 (comment 2432239)

通过使 gettext:po_to_json 命令适配 Jihu，解决 locale/en/app.js 等文件缺失的问题。这里都是 revert 掉之前的 commit，并没有新增的代码。

Why

在 SaaS 部署过程中，执行了 bundle exec rake gitlab:assets:compile 命令来生成 app.js 等文件，其中的核心代码是

Gitlab::TaskHelpers.invoke_and_time_task('gettext:po_to_json')

该命令之前有对 Jihu 的适配，但在最近的 MR 中被误删除了，导致不再生成 locale/en/app.js 等 Jihu 特有的文件。

Screenshots or screen recordings

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How to set up and validate locally

1. 手动删除 locale/en/app.js

   $ rm app/assets/javascripts/locale/en/app.js

2. 执行命令 rake gitlab:assets:compile, 期望重新生成 app.js

   $ rake gitlab:assets:compile
   $ ls app/assets/javascripts/locale/en/app.js

MR acceptance checklist

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