URBNG-320 - [WP] Implement mock-up database (UnitOfWork, Repository, Database), URBNG-321, URBNG-322, URBNG-317

URBNG-320 - [WP] Implement mock-up database (UnitOfWork, Repository, Database),

URBNG-321 - [WP] Provide unit tests for mock-up database, 

URBNG-322 - [WP] Replace in unit tests real database to mock-up implementation

URBNG-317 - [WP] Provide unit tests for storage relationships between Game <-> Task <-> ABCDPossibleAnswers (including interface implementation of relations)