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/forms.py

@@ -290,7 +290,7 @@ class EmailTemplateForm(BaseTemplateForm):
 class LetterTemplateForm(EmailTemplateForm):
 
     subject = TextAreaField(
-        u'Title',
+        u'Main heading',
         validators=[DataRequired(message="Can’t be empty")])
 
     template_content = TextAreaField(