Bug 23198: Wrong icons/paths in 18.11 for self check modules

Bug 23198: Wrong icons/paths in 18.11 for self check modules

This patch amends the CSS in self checkout and self check-in modules so
that paths to icon images are correct.

To test, apply the patch and make sure the WebBasedSelfCheck and
SelfCheckInModule preferences are enabled.

In self-checkout and self-check-in, confirm that the following icons
appear correctly:

 - "Help" (?) icon in the upper right.
 - "Finish" (stop sign) icon -- Shown to logged-in self checkout users
   from the start. Shown to self-check-in users after they check in an
   item.
 - "Cancel" (X) icon -- Shown for instance when you try to check out an
   item which is already checked out to you and can be renewed.
   - Also in this dialog: "Return" (Book with arrow) and "Renew"
    (refresh icon).

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>