Incorporate calendar_dates.txt into trip inference logic

[owades]

Update python and js trip inference to allow for just calendar.txt, just calendar_dates.txt, or both files to be used to describe service dates.

calendar_dates.txt often contains holiday-specific service additions/removals. Without incorporating this to trip inference, it will fail to recognize which routes should be running on, say, Christmas Eve.

The web app drop-downs already contain this info.