Use zero instead of 9999-12-31 for deadline
Fix#32291
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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>
Follow #32383
This PR cleans up the "Deadline" usages in templates, make them call
`ParseLegacy` first to get a `Time` struct then display by `DateUtils`.
Now it should be pretty clear how "deadline string" works, it makes it
possible to do further refactoring and correcting.
