Allow events to have a "TBD" or "All Day" time

[cassidoo]

Currently they must have a specific time range within the one day, which is limiting for future events, and limiting for all day, multi-day conferences. At present, if anything except a specific UTC time range is entered, the event does not appear on the site.

[karasowles]

Thank you @cthow83 for the PR allowing a TBD time for events! #302