Calendar: better styling for disabled days

[bill]

Currently the disabled day style looks like the style for days from the previous/next month, which presumably you _can_ select. Really need an X over each disabled day.

Originally I thought we need to change the template but maybe we can just do it with a background-image, and even though the X will be behind the number it will look the same as if it was in front of the number, assuming they are the same color.

[dante]

reminder to put example of calendar with disabled days in test_Calendar.html

[dante]

(In [10214]) fixes #4004 - disabled day node exposed in tundra now. Adding image and code to support the big 'X' over/under the label, but commented out and insdead of text-decoration:line-through (will scale with text size, etc). cursor reverts when hovering disabled day as well.