Add date range selector to DAD

[jeancochrane]

Update the DAD to add a date picker allowing the user to select a range for the data. The date picker should:

• Allow the user to select a single date
• Allow the user to select a range of dates
• If feasible, allow the user to select multiple discontinuous dates and ranges
• Display the selected dates in formatted plaintext underneath the chart
• Display the count of matching observations underneath the chart
• Update the chart to only show observations that intersect with the selected range

Depends on #96.