Bug: File gets uploaded even if it has invalid format

[RishabhJain2018]

Verify if this bug still exists, if yes, fix it :)

• When a user tries to create a challenge, if he uploads an invalid file ( zip without YAML) or any other format, the user gets an error but the files are stored in challenge configurations.
• This can result in large consumption of disk space and can be exploited by the users.

[Jk-karunadu]

@RishabhJain2018 working on it

[Jk-karunadu]

@RishabhJain2018 could you please specify that , if this issue is related to invalid upload due to missing of yml file while creation of challenge , Cloud-CV/EvalAI-Starters#99 this PR resolves that issue , by checking yml file present or not , previously challenges can be created using challenge_config.zip` by uplouding this in create challenge , but that is not available now

[Jk-karunadu]

we are only allowed to create challenge using github , now not allowed using challenge_config.zip

[RishabhJain2018]

Thanks @Jk-karunadu! Closing this since we had removed that feature.