Bug 31879: Convert mainpage.css to SCSS

Bug 31879: Convert mainpage.css to SCSS

This patch creates a new SCSS file to be used to generate mainpage.css,
the stylesheet used specifically for the staff interface home page.

The patch makes visible changes in only two places: Removing borders
from two elements to make them consistent with the new staff interface
design:
 - div.pending-info, where pending actions like patrons requesting
   modifications are shown.
 - div#area-userblock, where the contents of the IntranetmainUserblock
   system preference are displayed.

Otherwise the changes are all for the purpose of properly nesting
elements according to SCSS rules and other fixes related to stylelint
rules.

To test, apply the patch and rebuild the staff interface CSS
(https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_client).

View the main page of the staff interface. Everything should look like
it did before except for those elements which have had their borders
removed.

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