Make x-calendar/x-datepicker responsive about brick

[ldoubleuz]

As of bd2283e, calendar day widths are now set with percentages, allowing modification of the .month's width to affect day widths.

Although this still requires a set width on individual months, it is easier to calculate a single overall width instead of the width of each day. The width is set on months instead of x-calendar, as we may want calendars with more than one month displayed to stretch horizontally.

Leaving this open in case we come across a better responsive way to do this later.