Refactor server structure

[Jiahe Yan]

Currently

• A lot of outdated dependencies
• Celery integration is through various shell scripts etc, which makes the top directory very messy
• The connection between the Celery module and Meow backend is very weak (Celery simply queries the database every period of time to look for posts that can be sent) -> no reason to keep the two in the same repository

Proposal:

• Clean up the top directory and merge shell scripts into the Dockerfile
• Move Celery to an independent service repository
• Switching to a more modern Celery version (or use Airflow to replace Celery)