Lengthy end_date display

[rtanen]

Events with defined start_date and end_date fields currently will display as follows:

May 4, 2017, noon
May 4, 2017, 3pm

This gets annoying and confusing when the the year, month, or day fields are redundant. Ideally, end times would be shown as something without redundant data, like "May 4, 2017, noon to 3pm" or "May 4--7, 2017" or "May 4 -- June 1, 2017" or something.

I'm not sure what a good way to work time data into the last two items is, but we should also figure out how to do that.