Add Prize Money to Challenges

### Description

Currently it is difficult to find know which challenges have prizes, and if yes, what are the first/second/thrid prizes, etc. This feature will help us understand which challenges on EvalAI have prizes. The idea is to add a new prize money section here:

There are several components to be designed here:
1. The idea is to create a new model `ChallengePrizes` with the following details:
```
- challenge_pk
- amount
- rank
```
2. The challenge prizes will be a new section in the challenge configuration. Example:
```yaml
prizes:
- rank: 1
amount: "1000USD"
- rank: 2
amount: "500USD"
```
3. This will also require you to add changes to the [challenge validation script](https://github.com/Cloud-CV/EvalAI/blob/19130726790e4cb79b0d2e8603dd6817b16dba69/apps/challenges/challenge_config_utils.py#L223) to ensure people put appropriate values for prizes.
4. Add a new attribute `has_prizes` in the Challenge model.
5. You will also need to modify the `create_or_update_github_challenge` function to update the challenge and the entries.
7. Display the prizes for the challenge if the challenge has prizes. This will require you to modify the frontend.

### Pre-requisites/Learnings
This issue essentially requires background on a few things:
- An understanding of the [challenge configuration](https://evalai.readthedocs.io/en/latest/configuration.html).
- A deep understanding of [EvalAI challenges](https://evalai.readthedocs.io/en/latest/host_challenge.html).
- An understanding of [our frontend](https://github.com/Cloud-CV/EvalAI/tree/master/frontend).
- How the [challenge validation](https://github.com/Cloud-CV/EvalAI/blob/19130726790e4cb79b0d2e8603dd6817b16dba69/apps/challenges/challenge_config_utils.py#L223) works.
- How [GitHub Challenge Creation](https://github.com/Cloud-CV/EvalAI/blob/fbfc7f04cf8570704fd41808c0c5f5cfed518505/apps/challenges/views.py#L3298) works.

### How to Approach
- You can start by setting up EvalAI locally. Set up the dev version locally (preferred) using the steps [here](https://github.com/Cloud-CV/EvalAI/tree/master#installation-instructions).
- Add the new models, the new attribute to the backend.
- Update the challenge configuration, the validation and the create/update method to handle this.
- Play around with the the frontend, and create a PR. Please add screenshots of the changes.
- Reach out to us regarding any issues that you face or if you need any help with understanding any of the above.