Bug 37980: Style corrections for installer and onboarding following Bootstrap 5 update

Bug 37980: Style corrections for installer and onboarding following Bootstrap 5 update

This patch makes a couple of CSS updates to the installer and and
onboarding pages:

1. Add more Bootstrap imports to the installer SCSS. I included a
   pared-down selection of components to save file size. I should have
   included more.
2. Add a temporary fix for the fact that the Font Awesome checkbox icons
   aren't working correctly. According to the docs the "empty check box"
   icons should look like a square border, not a solid square. They
   don't, and I've included a temporary fix for it.

To test apply the patch and rebuild the staff interface CSS.

- Drop and recreate the Koha database. Run 'restart_all' if necessary.
- Navigate to the staff interface. You should be presented with the
  installer login. The form should be centered on the page.
- Log in. In the list of steps displayed on the "Choose your language"
  page you should see square icons resembling empty checkboxes (not
  solid black squares).
- Select your language and proceed to the next step. The "Choose your
  language" line should now have a green icon of a checked checkbox.
- Follow the rest of the installer and onboarding steps to confirm that
  there aren't other unreported style issues.

Signed-off-by: Phil Ringnalda <phil@chetcolibrary.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>