Require start and end dates for sections

[halturner]

The "Active" filter view relies on dates, and if we don't require them, most courses cannot be captured by that filter.

It's also best practice to have start and end dates for every section.

[eliknebel]

Consider making open-ended, long running enrollment courses "opt-in" with some sort of checkbox, but otherwise require start and end date to be specified for all courses

[eliknebel]

https://eliterate.atlassian.net/browse/MER-1717