Add remaining endpoints for PDFs and PNGs

Right now we can show what a letter template looks like as a PDF or PNG.

This commit completes the work so this is also possible when:

- showing a template with the placeholders replaced
- showing any version of a template

Also removes dependency on `Exception().message`, which was deprecated
in Python 2.6. See
97f82d565f
for full details.

app/main/forms.py

@@ -96,7 +96,7 @@ class UKMobileNumber(TelField):
         try:
             validate_phone_number(self.data)
         except InvalidPhoneError as e:
-            raise ValidationError(e.message)
+            raise ValidationError(str(e))
 
 
 def mobile_number():