Simple Date Popup

An adaptation of the Date Popup sub-module of the Date module. Gets rid of the fieldset, so that the text field from which the calendar pops up is actually rendered as a simple text field. There is no timepicker, no "to" date - this is for simple date entry only.

Other improvements:
Initial display of the date on the form is in the correct, selected format.
Validation is less stringent - any date that the PHP DateTime class recognizes as a date will be treated as valid.
At window.matchMedia("(max-width: 42em)").matches, the included javascript drops the popup and changes the text field to a date field for a better mobile experience (note that this happens on load, and does not adjust to subsequent changes in window size).

TODO:
Add option to use HTML5 date field.
Set the breakpoint through the UI.