BUG: loffset argument not applied for resample().count() on timeseries
- [x] closes #12725
- [ ] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

The code to do this already existed in the `_downsample` method, which is called when using functions like `mean`. `max`, etc., but there was nothing in the `_groupby_and_aggregate` method used for `count` and `size`. If we pull out the offset code from the `_downsample` method into a separate method, we can reuse it without duplicating it.

@onesandzeroes perfect!
