[FIX] website_slides: fix webiste menu being not clickable in lesson fullscreen

Before this commit, the navigation menu was not clickable because of higher
z-index of the slides fullscreen menu. This commit fixes that by correctly
setting z-index, using the standard variable to avoid custom values.

task-2036877

closes odoo/odoo#35107

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

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

 
 .o_wslides_fs_main {
     @include o-position-absolute(0,0,0,0);
-    z-index: 2000;
+    z-index: $zindex-dropdown - 1;
     background-image: linear-gradient(120deg, $o-wslides-color-dark2, $o-wslides-color-dark3);
 
     .o_wslides_slide_fs_header {