Rename CSS classes for consistency

This prefixes everything to do with areas/the map with `area-list`, so
from looking at one element you know which `.scss` file will contain the
relevant code.

app/templates/views/broadcast/view-message.html

@@ -138,7 +138,7 @@
     {% endif %}
   {% endfor %}
 
-  <div class="map-details">
+  <div class="area-list-map-details">
     {{ govukDetails({
       "summaryText": "Show map of areas",
       "html": map(broadcast_message)