Bug 29112: Module navigation sidebars in staff now show bullet points

Bug 29112: Module navigation sidebars in staff now show bullet points

Bug 27873 removed an image file which was still being referenced in the
staff interface CSS, intended to replace the default bullet point in
sidebar menus like Administration and Tools pages have.

This patch creates a new variable in the SCSS "mixins" file so that the
SVG image file created by Bug 27873 can be reused, and corrects the CSS
for the affected menus.

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).

- Go to Tools -> Patron lists
  - The sidebar menu should be styled with little arrows instead of
    the default HTML bullets.
- Check the same on an Administration page, e.g. Administration ->
  Libraries.
- Check that there hasn't been a regression on the checkout and patron
  detail views:
  - Check out to a patron who has one or more messages on their account,
    e.g. overdues, return claims, holds waiting, notes, messages, etc.
  - The display of these list items should be the same: An arrow bullet
    point instead of the default.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@iki.fi>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>