Back end: Implemented progress resetting functionality. Resolves #99.
Back end: Implemented question deletion. Resolves #105.
Back end: Implemented chapter deletion. Resolves #104.
Back end: Implemented question deletion during chapter deletion. #104, #105.
Back end: Implemented book deletion. Resolves #72.
Back end: Started refactoring object deletion. #110
Back end: Refactored object deletion from the application. Resolves #110.
Back end: Fixed exception syntax error in the BookFragment.