Use color-pick-contrast to get text colors

We were defining (for instance) white text against the `$brand`
background. That meant that, if somebody customized the `$brand` color
so it used a light color, they had to customize the text color as well
in order to guarantee proper contrast between text and background
colors.

So we're using `color-pick-contrast` instead, which means we don't have
to manually calculate whether white or black will be the color which
makes the text more readable.

app/assets/stylesheets/admin/machine_learning/setting.scss

@@ -2,7 +2,7 @@
 
   .card-divider {
     background: $primary-color;
-    color: $white;
+    color: color-pick-contrast($primary-color);
 
     h3 {
       margin-top: 0;

app/assets/stylesheets/admin/menu.scss

@@ -2,7 +2,7 @@
   background: $sidebar;
   background: linear-gradient(to bottom, $sidebar 0%, #488fb5 100%);
   border-right: 1px solid $border;
-  color: $white;
+  color: color-pick-contrast($sidebar);
 
   ul {
     list-style-type: none;