Series.value_counts: Preserve original ordering
Ensure that value_counts returns the same ordering of the indices than the input object
when sorting the values no matter if it is ascending or descending.

- [x] closes #12679
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
- [ ] About 10 tests of the testsuite are currently failing and I hope to find a fix for them soon as well. Wanted to put up this PR now already to get feedback on the current implementation as well.