feat(datepicker): allow closing of calendar popup with the Escape key

Improve keyboard usability and accessibility of the directive. Calendar popup will now close when
the Escape key is pressed while keyboard focus is on the text input OR a button within the popup.
Previously, the only way to close the calendar popup with a keyboard was to Tab through to and
activate the Close button (potentially forcing the user to tab through 30+ items). Calendar popup
will restore when a user starts typing in the text input, and when focus returns to the text input
after being focused on other page element.