BUG: Fix Period and PeriodIndex support of combined offsets aliases
- [x] closes #13730
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Essentially, makes sure any `freq` string passed to a `Period` or `PeriodIndex` goes through [`_Period._maybe_convert_freq()`](https://github.com/pydata/pandas/blob/master/pandas/src/period.pyx#L682-L697), which calls [`to_offset()`](https://github.com/pydata/pandas/blob/master/pandas/tseries/frequencies.py#L389-L451), which is where the logic for combining aliases is.

All the examples in #13730 result in the correct output, and all existing tests pass. I have not written any new ones yet &mdash; I first wanted to get the opinion of the maintainers.

This PR builds on #13868 (without it, some existing tests fail).
