Call letter subjects ‘main heading’

This is a term that one of our research participants used to describe
the big bold text that starts each letter. I think it’s quite a nice
plain english term for it.

Also changes the formatting guidance to use the word heading instead of
title, for consistency.

app/main/views/templates.py

@@ -171,9 +171,9 @@ def add_template_by_type(service_id):
             blank_letter = service_api_client.create_service_template(
                 'Untitled',
                 'letter',
-                'Content',
+                'Body',
                 service_id,
-                'Title',
+                'Main heading',
                 'normal',
             )
             return redirect(url_for(