Follow https://github.com/go-gitea/gitea/pull/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.
(cherry picked from commit 259811617ba15c77ddd89360178a59251d611af2)