Register users into redis

Right now, users' data is stored inside python dictionary. Replace this mock solution with communication with redis. Use async library: aioredis. Implement connection to redis as fastapi dependency using yield.

Save users under username (or email), hash passwords, etc.

Resources: https://youtu.be/oNz_uXjOCpo https://redis.io/ https://aioredis.readthedocs.io/en/latest/ https://fastapi.tiangolo.com/tutorial/dependencies https://fastapi.tiangolo.com/tutorial/dependencies/dependencies-with-yield/?h=y