Disable all scheduled tasks with a setting

[mbertrand]

What are the relevant tickets?

N/A

Description (What does it do?)

Adds a new optional setting (CELERY_BEAT_DISABLED), default of False, that if True will set CELERY_BEAT_SCHEDULE={}

I decided to add this because some of the daily scheduled tasks can take awhile to complete and interfere with development, testing, and code reviews.

How can this be tested?

• Run docker compose up as usual.
• In a shell:

    from django.conf import settings
    settings.CELERY_BEAT_SCHEDULE

  You should get back a dict with multiple scheduled tasks
• Add CELERY_BEAT_DISABLED=True to your backend.local.env file
• Restart containers, then rerun the shell code. You should get back an empty dict,