Update backlog linechart hovertext to show the date if the chart frequency is 'D' or daily.

[brl1906]

1. find open_vs_completed chart generating function in charts.py
2. add functionality that determines if the frequency parameter is 'D' or daily and if so,
   change the trace hovertext so that it displays the date and number open and closed in a pretty format , for example:

• May 3, 2017

• opened: 45

• May 3, 2017

• completed: 44