Localized DateTime ambiguous handling

[maennchen]

          > does that mean most functions in DateHelper need to return either one or two dates?

Probably not:

If you have the setting on both, we can just return the first occurrence. When we look for the next candidate date, we should automatically reach the later occurrence.

So the logic could be something like this:

• if result ambiguous and setting is earlier: return earlier
• if result ambiguous and setting is later: return later
• if result ambiguous and setting is both: return earlier or later depending on adjusted

Originally posted by @maennchen in #129 (comment)

Needed for quantum-elixir/quantum-core#407