Set up celery

[paulapreuss]

To allow to run scheduled jobs again and fix #381. Used to be handled with Django-Q but being updated due to Django Q being outdated and to share a similar setup with other repositories for easier maintenance. The configuration is mostly identical to what is used on the offgridplanner. I also fixed some errors on the dependabot config in this PR, but it is unrelated to the celery function.

@Bachibouzouk there is already an instance of beat/celery worker running on the server watching the sandbox server, all seems to be working. Simulations can be started, return to the projects page and the simulation status will be updated automatically on refresh when it changes.

Once we merge these changes into main, what should need to be done on the server is:

• change the branches from which to deploy the celery instances on the server (from feature/celery-beat to main)
• point to the production database instead of the sandbox one
• set up the webhooks for the celery instances to also update automatically on push