Mailing: Add support for a from field

Add support to the mailing profiles for specifying a "from"
address. Closes GitLab issue #522.

This commit adds a new backend class property "class_fields"
which differs from the normal "fields" property. The "class_fields"
property specifies which of the backend fields will be used to
initialize a backend's driver class. This is to avoid passing
fields that the driver doesn't expect and getting an error.

When sending emails, the "send" method will attempt to get
a "from" key from the backend data and use that when sending
emails. If no "from" key is found a None is passes. Django's
behavior in this situation dictates that the "from" value will
then be taken from the DEFAULT_FROM_EMAIL setting.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/mailer/classes.py

@@ -48,6 +48,11 @@ class MailerBackendBase(object):
     class_path = ''  # Dot path to the actual class that will handle the mail
     fields = {}
 
+    @classmethod
+    def get_class_fields(cls):
+        backend_field_list = getattr(cls, 'fields', {}).keys()
+        return getattr(cls, 'class_fields', backend_field_list)
+
 
 class MailerBackend(six.with_metaclass(MailerBackendMetaclass, MailerBackendBase)):
     @classmethod