Update side navigation active / hover contexts

This aligns closer to RailsAdmin 2.x.x, where the background of an element in the side nav changes when it's either active or hovered over - making the navigation itself easier to follow in the UI

src/rails_admin/styles/base/theming.scss

@@ -42,6 +42,16 @@ body.rails_admin {
     > li > a {
       padding: ($navbar-padding-y / 2) ($grid-gutter-width / 2);
 
+      &.active {
+        background-color: map-get($theme-colors, primary);
+        color: color-contrast(map-get($theme-colors, primary));
+      }
+
+      &:hover {
+        background-color: map-get($theme-colors, primary);
+        color: color-contrast(map-get($theme-colors, primary));
+      }
+
       &.nav-level-1 {
         padding-left: $grid-gutter-width;
       }