[notification] use an InternalManager object for notifications if we have an (email, lang) tuple
The non-uniform handling of CWUser vs (email, lang) tuple has caused too
many bugs; this change mostly unifies the two code paths. Closes #3381521
[req] if a user has a None property, fall back to the default
This can happen for InternalManager; the normal CWUser implementation of
property_value already has a similar fallback. Related to #3381521