Submission worker: Add successful_submission_flag==True condition before saving submission_output

- **Expected behaviour**:
If the submission evaluation is not succesful then the submission output in the database should not be updated.
_Example_: if the challenge splits in the output and the database don't match, this line should not run: https://github.com/Cloud-CV/EvalAI/blob/34cf4572d6938a5f16a4d97b1e8434ee5bb49d75/scripts/workers/submission_worker.py#L513
- **Observed behaviour**:
The submission output in the database is updated irrespective of the contents of the evaluated submission output.

@RishabhJain2018