dashboard/app: close fix-bisected bugs as fixed rather than obsolete

[dvyukov]

"WARNING in cow_file_range_inline" report:
https://syzkaller.appspot.com/bug?extid=858534c396b0cdd291d2
https://lore.kernel.org/all/[email protected]/

It was correctly fix-bisected, but then closed as obsolete. If it's indeed fixed, it's expected that it won't happen and will be closed as obsolete soon.
Since we don't systematically handle fix bisection results, I think it's more reasonable to close such bugs as fixed with the commit (at the time it would be currently obsoleted, to give any human moderators time to act).

[dvyukov]

Another example where the same happened:
https://syzkaller.appspot.com/bug?extid=59a71007ccac79e8bb69

We could count how many such cases we have, and potentially fix up the data base.