Allow selecting date range on single calendar

[youngj]

Currently the DateTimePanel control has separate input controls for Start Date and End Date. When the End Date changes, the Start Date will automatically be changed to be on or before the End Date. This user interface is cumbersome to use because selecting a single date may require typing the date in both fields, and selecting a date range may require clicking dates in two separate calendars. It would be a better user experience if there was a single control for the date range with a single calendar. For an example, see Uber Movement's date range control: https://movement.uber.com/explore/san_francisco/travel-times/query