Bug 28044: Increase specificity of calendar tables CSS

Bug 28044: Increase specificity of calendar tables CSS

This patch adds increased specificity to the CSS which should add colors
to the tables of holidays. The default DataTable CSS was overriding the
custom color-coding.

This patch also moves the calendar CSS into a separate file.

To test, apply the patch and go to Tools -> Calendar.

If necessary, add at least one of the following:
- Unique holiday
- Holiday repeating weekly
- Holiday repeating monthly
- Holiday exception

In the table listing each kind of holiday, the color of the table header
should match the colors in the "Key" section under Hints. The color
should be correct regardless of how the table is sorted.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>