[FIX] website_slides: fix quiz completion blocking users in fullscreen

Once a quiz is completed in elearning fullscreen view a pop-up is displayed.
Due to z-index it makes a black overlay on full-screen, so that we can not
click anywhere on the screen.

In this commit we update z-index value fix the issue and make screen clickable.
Website menu is also still clickable.

Task: 2044221

addons/website_slides/static/src/scss/slides_slide_fullscreen.scss

@@ -1,7 +1,7 @@
 
 .o_wslides_fs_main {
     @include o-position-absolute(0,0,0,0);
-    z-index: $zindex-dropdown - 1;
+    z-index: $zindex-dropdown;
     background-image: linear-gradient(120deg, $o-wslides-color-dark2, $o-wslides-color-dark3);
 
     .o_wslides_slide_fs_header {
@@ -114,4 +114,10 @@
             pointer-events: none;
         }
     }
-}
+}
+
+.modal-open {
+    > .modal-backdrop {
+        z-index: $zindex-dropdown - 1;
+    }
+}