Task #349 - API fails when DELETE-ing an userstory

Link - https://kaleidos.taiga.io/#/project/taiga/tasks/349
Add invitations and empty projects to the sample_data command
Fix an error
WIP: Simple proposal of permission management
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Import some rest framework parts with own changes.
Add views.py
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Adding first base_api_permissions test
Some tests fixed
test_userstorage_api working