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.
Reorganize various CSS files for clarity, group together by subdirectory
in `index.css`. This reorders some of the rules, but I don't think it
should introduce any issues because of that.
