Refactored tasks.py to split out letters_pdf tasks

- Added has_permission helper in models.py to check permission in service
- Moved letters pdf tasks to separate file
- Moved letters pdf tests to own file

app/models.py

@@ -283,6 +283,9 @@ class Service(db.Model, Versioned):
         default_letter_contact = [x for x in self.letter_contacts if x.is_default]
         return default_letter_contact[0].contact_block if default_letter_contact else None
 
+    def has_permission(self, permission):
+        return permission in [p.permission for p in self.permissions]
+
 
 class AnnualBilling(db.Model):
     __tablename__ = "annual_billing"