Add Time to the challenge schedule

[jmgasper]

On the edit challenge screen, we show the challenge schedule as a Gantt chart, which shows the days, but it doesn't show time anywhere on the chart. The time isn't on the popups on the Gantt chart or on the challenge schedule text around the chart.

We need to:

1. Add time information to the Gantt chart popup
2. Add the start time and date and end time and date above the Gantt chart to show the user exactly when a challenge is starting and is anticipated to finish.