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>

docs/releases/3.2.rst

@@ -77,6 +77,8 @@ Backward incompatible changes
 Bugs fixed or issues closed
 ---------------------------
 
+* :gitlab-issue:`522` Office 365 SMTP
 * :gitlab-issue:`539` Setting for default email sender is missing
 
+
 .. _PyPI: https://pypi.python.org/pypi/mayan-edms/