Add status query managers that inherit methods of a custom queryset

[EliasCampos]

Problem

Please see #486

Solution

In add_status_query_managers signal handler, add creation of manager class, that inherits QueryManager and takes methods of a queryset, returned by .get_queryset() method of a default manager. Status managers, added to a model, are instances of the created manager.

Commandments

• Write PEP8 compliant code.
• Cover it with tests.
• Update CHANGES.rst file to describe the changes, and quote according issue with GH-<issue_number>.
• Pay attention to backward compatibility, or if it breaks it, explain why.
• Update documentation (if relevant).

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@57d26ee). Click here to learn what that means.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #487   +/-   ##
=========================================
  Coverage          ?   94.58%           
=========================================
  Files             ?        6           
  Lines             ?      757           
  Branches          ?        0           
=========================================
  Hits              ?      716           
  Misses            ?       41           
  Partials          ?        0           

Impacted Files	Coverage Δ
model_utils/models.py	98.93% <100.00%> (ø)

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