Swap headings for list items in template list

I'm mainly making this change because it's useful
for the CSS that styles the hint text when the
link is focused for the link to have no parent
container.

That being said, there isn't really enough content
underneath these headings to justify them as it is.
I've wrapped them in a list instead because:
- they're structured like a list
- we already called them a `template-list`

This commit also replaces the `message-type` class
on the paragraph below where the headings went,
for consistency. It also removes the CSS for that
class as I couldn't find anywhere else that used
it now.

app/assets/stylesheets/components/message.scss

@@ -56,14 +56,6 @@ $message-type-bottom-spacing: govuk-spacing(4);
 
   }
 
-  &-type {
-    position: relative; /* needs to be non-static to have a z-index above the link :before element */
-    color: $govuk-secondary-text-colour;
-    margin: 0 0 $message-type-bottom-spacing 0;
-    padding-left: 0;
-    pointer-events: none;
-  }
-
 }
 
 .template-list {