Fix#2616
This PR adds a new sort option for exclusive labels.
For exclusive labels, a new property is exposed called "order", while in
the UI options are populated automatically in the `Sort` column (see
screenshot below) for each exclusive label scope.
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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Replace #26661, fix#25979
Not perfect, but usable and much better than before. Since it is quite
complex, I am not quite sure whether there would be any regression, if
any, I will fix in first time.
I have tested the related pages many times: issue list, milestone issue
list, project view, user issue list, org issue list.
