Provide a default if FILE_UPLOAD_TEMP_DIR is not set

[yakky]

Fix #87

[coveralls]

Coverage decreased (-0.08%) to 90.27% when pulling 4d300a4 on hotfix/temp_dir into 871f075 on release/0.9.x.

[codecov-io]

Codecov Report

Merging #90 into release/0.9.x will decrease coverage by 0.18%.
The diff coverage is 60%.

@@                Coverage Diff                @@
##           release/0.9.x      #90      +/-   ##
=================================================
- Coverage           84.1%   83.91%   -0.19%     
=================================================
  Files                  7        7              
  Lines                736      740       +4     
  Branches             125      126       +1     
=================================================
+ Hits                 619      621       +2     
- Misses                71       72       +1     
- Partials              46       47       +1

Impacted Files	Coverage Δ
djangocms_helper/base_test.py	89.85% <60%> (-0.79%)	⬇️

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