Add grey lines to visually show level of nesting

Uses some CSS to draw some grey lines to show which radios descend from
which.

I don’t feel like the intentation is enough, and it looks a bit messy
because the circles of the radio buttons don’t have straight edges
easily suggest visual alignment.

Copies the design from conditionally revealing content in the design
system:
https://design-system.service.gov.uk/components/radios/#conditionally-revealing-content

Implementation is done with pseudo elements, because borders can’t be
positionned exactly enough.

app/assets/stylesheets/components/pill.scss

@@ -57,11 +57,11 @@
     border: 2px solid $black;
     outline: 1px solid rgba($white, 0.1);
     position: relative;
-    z-index: 1000;
+    z-index: 10;
     color: $text-colour;
 
     &:focus {
-      z-index: 10;
+      z-index: 1000;
       outline: 3px solid $yellow;
     }
 

app/assets/stylesheets/components/radio-select.scss

@@ -57,3 +57,60 @@
   }
 
 }
+
+.radios-nested {
+
+  margin-bottom: 10px;
+
+  .multiple-choice {
+
+    $circle-diameter: 39px;
+    $border-thickness: 4px;
+    $border-indent: ($circle-diameter / 2) - ($border-thickness / 2);
+
+    float: none;
+    position: relative;
+
+    &:before {
+      content: "";
+      position: absolute;
+      bottom: 0;
+      left: $border-indent;
+      width: $border-thickness;
+      height: 100%;
+      background: $border-colour;
+    }
+
+    label {
+      float: none;
+    }
+
+    [type=radio]+label::before {
+      // To overlap the grey inset line
+      background: $white;
+    }
+
+    ul {
+      // To equalise the spacing between the line and the top/bottom of
+      // the radio
+      margin-top: 5px;
+      margin-bottom: -5px;
+    }
+
+    .block-label-hint {
+      &:after {
+        // Adds an little extra segment of line alongside the ‘current folder’
+        // hint so that it extends all the way down to the next radio
+        content: "";
+        position: absolute;
+        bottom: -5px;
+        left: $border-indent;
+        width: $border-thickness;
+        height: 25px;
+        background: $border-colour;
+      }
+    }
+
+  }
+
+}