Add command to download all submissions of a challenge

### Description

We need a cli command that will allow users to download all submissions of a challenge. The command should handle two cases:

1. If a user is host of a challenge download all submissions made to the challenge for a phase
2. If a user is a participant download submissions only made by the team of user to the challenge for a phase.

Required backend APIs are already available for this feature [here](https://github.com/Cloud-CV/EvalAI/blob/master/apps/challenges/views.py#L1669).