fix wrong usage of mix(), fixes #1868

Closes #1868

Merge request studip/studip!1222

resources/assets/stylesheets/scss/big-image-handler.scss

-$bg-big-image-handler: mix($black, $white, 30%);
+$bg-big-image-handler: fade-out($black, 0.3);
 
 .oversized-image {
     cursor: zoom-in;
@@ -31,7 +31,7 @@ $bg-big-image-handler: mix($black, $white, 30%);
         background-repeat: no-repeat;
         background-size: contain;
         border: 1px solid $white;
-        box-shadow: 0 0 20px mix($black, $white, 50%);
+        box-shadow: 0 0 20px fade-out($white, 0.5);
         display: block;
         max-width: 98%;
         max-height: 98%;

resources/assets/stylesheets/scss/dialog.scss

 .ui-widget-overlay {
-    background: transparentize($base-gray, .2);
+    background: fade-out($base-gray, 0.2);
     opacity: 1;
     position: fixed;
 }

resources/assets/stylesheets/scss/studip-overlay.scss

@@ -16,7 +16,7 @@
     bottom: 0;
 
     &-local {
-        background-color: mix($light-gray-color, $white, 50%);
+        background-color: fade-out($light-gray-color, 0.5);
         position: absolute;
     }
 

resources/assets/stylesheets/scss/studip-selection.scss

@@ -31,7 +31,7 @@
     }
 
     li.empty-placeholder {
-        color: mix($text-color, $white, 30%);
+        color: fade-out($text-color, 0.3);
 
         &:not(:only-child) {
             display: none;