Bug 22638: (follow-up) Convert SCI CSS to SCSS

Bug 22638: (follow-up) Convert SCI CSS to SCSS

This patch makes a number of follow-up changes, including:

 - Covert remaining icons to Font Awesome
 - Convert CSS to SCSS
 - Move SCSS which is common to both the OPAC and SCI into separate
   files to be including in each during compilation.
 - Make minor correction to opac.scss to fix SCSS build warning

To test, apply the patch and rebuild the OPAC CSS. Clear your browser
cache if necessary.

- Enable the self checkin module using using the SelfCheckInModule
  system preference.
- Open the self checkin interface and test the checkin process.
  Everything should look correct.
  - After submitting barcodes for checkin, the "Finish" button should
    show a Font Awesome icon instead of an image icon.
- Enable multiple translations and set the OpacLangSelectorMode
  preference to show the language menu in the footer.
- Confirm that the language menu looks correct.
- View the regular OPAC to make sure the changes to opac.scss didn't
  result in unintended changes to the interface.

Signed-off-by: Liz Rea <wizzyrea@gmail.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>