I18N - scan JH folder to generate po file

Background

we need enable gettext rake task to scan jh folder to generate po file instead of manually modify it.

1. scan JH folder to get all translation
2. keep JH namespace translation
3. read existing JH po file to get existing override key
4. merge them and write JH po file

Related Links

1. Pot definition
2. GitLab i18n externalization doc
3. Rake::Task["gettext:po:update"] is defined at gettext/tools/task.rb#L475