Bug 28978: Convert installer CSS to SCSS

Bug 28978: Convert installer CSS to SCSS

This patch creates a new SCSS file, installer.scss, from which
installer.css will be compiled.

Most of the resulting CSS is unchanged, but some minor sections were
removed because they were obsolete. The jQueryUI-specific section isn't
fully converted to SCSS because it's going to go away with the addition
of Flatpickr.

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

- Confirm that koha-tmpl/intranet-tmpl/prog/css/installer.css is
  updated.
- Go through the complete web installation process, including
  onboarding, to confirm that everything is styled as before.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>