Add challenge domains to challenges

### Description
Currently it is difficult to find challenges by domain (say CV, NLP, RL), etc. This issue is a first step towards that direction where we add the domain a particular challenge belongs to and display it on the challenge page in this section:

The challenge domain must be made a part of the challenge configuration and the [EvalAI-Starters](https://github.com/Cloud-CV/EvalAI-Starters) repository.
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 domains. Please create a PR with these dummies - `[Computer Vision, Natural Language Processing, Reinforcement Learning]`, before to converge to the set of domains we might want to include.

### 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 high-level 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 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 a new field to the challenge model in Django.
- Play around with the challenge configuration and the frontend, put up screenshots.
- Update the [challenge validation script](https://github.com/Cloud-CV/EvalAI/blob/19130726790e4cb79b0d2e8603dd6817b16dba69/apps/challenges/challenge_config_utils.py#L223) to check for the newly added field.
- Reach out to us regarding any issues that you face or if you need any help with understanding any of the above.