MDL-41792 core_calendar: fixed issues when using multiple calendar types

1) No longer assume that the end of the current month in Gregorian will be
the end of the month for the calendar type being used.
2) Need to take into account the hours and minutes a calendar type may vary
in order to generate timestamps.
3) When generating the previous and next month links start at the beginning
of the month, not the current day.
4) Need to convert to Gregorian when creating timestamps during calendar
exports.