Minor changes/improvements on exceptions tree.
Cosmetic fixes on project serializers.
Remove global project import from stars service.
Improve queries and transaction management for better testability.
Add method for attach stars count to projects queryset.
Cosmetic changes on stars __init__.py file.
Improved get_queryset methods on project api resources.

Attaching stars count to querysets.
Fix unhandled not found exceptions on get starscount on project serializer...

... using annotated stars count value instead of obtaining stars object.
Remove unnecesary Response usage on wiki page api resource.
Fix stars tests.
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WIP: splitted history mixin.
WIP: services and mixins.
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WIP: tmp
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