Improvements to date/time display for events

[bernhold]

Possible improvements in the display of dates and time in events (primarily in _includes/bsswt.html):

• If end-date is present but the same as date, ignore it in output
• Collapse the date range presentation to the tersest form. If years are common, don't repeat. If months are common, don't repeat
• Link time(s) to a service that facilitates time zone translation
  • e.g., https://arewemeetingyet.com/ as one possibility
• Accommodate multi-day tutorials with different times on each day
  • This would be a bigger lift. Hasn't happened in the history of our tutorials. Maybe defer until the need arises.