Do not call django.setup() multiple times, test pytest_configure etc order

[blueyed]

This is done in pytest_configure already (later), and allows for
projects to do stuff before Django is setup in their pytest plugin.

Fixes #531.

TODO:

• test(s)

The main problem here is that importing models in your conftest.py will not work anymore, because apps are not ready then yet.

I think we should ensure that pytest_load_initial_conftests can be used in a user's plugin, and that _setup_django does not call django.setup multiple times (#531).

[codecov-io]

Codecov Report

Merging #629 into master will decrease coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #629      +/-   ##
=========================================
- Coverage   92.46%   92.4%   -0.06%     
=========================================
  Files          32      33       +1     
  Lines        1751    1725      -26     
  Branches      146     143       -3     
=========================================
- Hits         1619    1594      -25     
  Misses         94      94              
+ Partials       38      37       -1

Flag	Coverage Δ
#dj110	84.57% <100%> (-0.18%)	⬇️
#dj111	86.66% <100%> (-0.15%)	⬇️
#dj18	85.27% <100%> (-0.28%)	⬇️
#dj19	84.46% <100%> (-0.18%)	⬇️
#dj20	84.86% <100%> (-0.17%)	⬇️
#dj21	80.75% <100%> (-0.23%)	⬇️
#mysql_innodb	84.86% <100%> (-0.17%)	⬇️
#mysql_myisam	84.69% <100%> (-0.18%)	⬇️
#postgres	88.23% <100%> (-0.12%)	⬇️
#py27	89.79% <100%> (-0.1%)	⬇️
#py34	84.46% <100%> (-0.18%)	⬇️
#py35	84.57% <100%> (-0.18%)	⬇️
#py36	85.33% <100%> (-0.17%)	⬇️
#py37	80.75% <100%> (-0.23%)	⬇️
#sqlite	86.78% <100%> (-0.14%)	⬇️
#sqlite_file	84.46% <100%> (-0.18%)	⬇️

Impacted Files	Coverage Δ
tests/test_initialization.py	100% <100%> (ø)
pytest_django/plugin.py	86.08% <100%> (-0.09%)	⬇️
pytest_django/fixtures.py	96.91% <0%> (-0.15%)	⬇️
tests/conftest.py	100% <0%> (ø)	⬆️
tests/test_fixtures.py	100% <0%> (+0.4%)	⬆️

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