Backport #22823
As part of administration sometimes it is appropriate to forcibly tell
users to update their passwords.
This PR creates a new command `gitea admin user must-change-password`
which will set the `MustChangePassword` flag on the provided users.
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Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Jason Song <i@wolfogre.com>
