make serializer configurable via django settings

[arneb]

Being able to change the serializer via Django Settings it is possible to implement advanced export/import schemes without touching djangocms-transfer itself.

Example: We subclassed Django's build-in python serializer and made it inline image data base64 encoded. Once the serializer is registered in Django (possible via Settings), we can use it in djangocms-transfer and are now able to export and import between different installations including images.

Other use-cases might be possible as well with custom serializers ...

[codecov-commenter]

Codecov Report

Merging #14 into master will increase coverage by 0.38%. The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
+ Coverage   29.20%   29.58%   +0.38%     
==========================================
  Files          10       10              
  Lines         363      365       +2     
  Branches       56       56              
==========================================
+ Hits          106      108       +2     
  Misses        257      257              

Impacted Files	Coverage Δ
djangocms_transfer/datastructures.py	48.38% <50.00%> (+1.72%)	:arrow_up:
djangocms_transfer/helpers.py	20.68% <50.00%> (+2.83%)	:arrow_up:

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[SteveGZr]

Bump

[viliammihalik]

@marksweb can you merge this one pls?

[SteveGZr]

Too bad this is still open.

@marksweb Would it be possible for you to review the changes?

[SteveGZr]

Thanks for merging!