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/templates/components/radios.html

@@ -19,10 +19,9 @@
     options,
     child_map,
     disable=[],
-    option_hints={},
-    top_level=False
+    option_hints={}
 ) %}
-    <ul{% if not top_level %} class="panel panel-border-narrow"{% endif %}>
+    <ul>
     {% for option in options %}
       {% if child_map[option.data] %}
         {% call radio(option, disable, option_hints, as_list_item=True) %}
@@ -47,7 +46,9 @@
   {% call radios_wrapper(
     field, hint, disable, option_hints, hide_legend
   ) %}
-    {{ radio_list(child_map[None], child_map, disable, option_hints, top_level=True) }}
+    <div class="radios-nested">
+      {{ radio_list(child_map[None], child_map, disable, option_hints) }}
+    </div>
   {% endcall %}
 {% endmacro %}