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/tests/literals.py

@@ -2,6 +2,7 @@ from __future__ import unicode_literals
 
 TEST_BODY_HTML = '<strong>test body</strong>'
 TEST_EMAIL_ADDRESS = 'test@example.com'
+TEST_EMAIL_FROM_ADDRESS = 'from.test@example.com'
 TEST_RECIPIENTS_MULTIPLE_COMMA = 'test@example.com,test2@example.com'
 TEST_RECIPIENTS_MULTIPLE_COMMA_RESULT = [
     'test@example.com', 'test2@example.com'