feature/EXT-1

[EugeneKovalev]

What:

• Allow setting a User model which differs from the common AUTH_USER_MODEL using LOGGING_USER_MODEL.
• If LOGGING_USER_MODEL is not set then use AUTH_USER_MODEL (in order not to spoil previous functionality)

Why:

• Currently _local.user = request.user.
  In rare occasions, request.user does not stand for django_users. This case might be represented as a result of merging 2 or more enormously large projects. Or gradual move from one type of DB to another. Or the interaction of multiple systems which use their own users but still it represents the same instance.
  As a result, there'd be multiple users tables in different DBs which are impossible to be merged into a single django_users at the moment.

To conclude, Django can work with multiple DBs then request.user can be represented not by AUTH_USER_MODEL but by something else.