Sprinkle letter throughout the app

Let users create/edit/delete letter templates.

Let them upload a CSV file or send a test against a letter template.

Big assumption at the moment is that addresses only have one line, and
therefore one column in the CSV file.

app/main/forms.py

@@ -276,6 +276,10 @@ class EmailTemplateForm(SMSTemplateForm):
         validators=[DataRequired(message="Can’t be empty")])
 
 
+class LetterTemplateForm(EmailTemplateForm):
+    pass
+
+
 class ForgotPasswordForm(Form):
     email_address = email_address(gov_user=False)