Bug 31765: Add page-section and bg-info classes to 'Notes'

Bug 31765: Add page-section and bg-info classes to 'Notes'

This patch adds the page-section and bg-info classes to the 'Notes'
section of the patron imports page.

As part of this patch, we update the base 'card' mixin to allow
background colors to be overwridden with the standard bg-* options
available in [bootstrap](https://getbootstrap.com/docs/4.0/utilities/colors/)

Test plan
1) Build your CSS in the usual way
2) Confirm the Notes block looks OK on the patron imports tool

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>