add support for enable_celery setting

[allynt]

started writing this code:

import functools
from .conf import app_settings

def conditional_task(task_fn):
    @functools.wraps(task_fn)
    def conditional_task_wrapper(*args, **kwargs):
        if not app_settings.ASTROSAT_TASKS_ENABLE_CELERY:
            return None
        task_fn(*args, **kwargs)
    return conditional_task_wrapper

But noted that the task still ran, it just didn't do anything if ASTROSAT_TASKS_ENABLE_CELERY was False.

---

Instead I can use a post_save signal on TaskSettings to somehow trigger disabling celery... maybe by re-configuring the celery app?

[allynt]

This is too f#@$ng! hard for now. I can just disable periodic tasks explicitly in the db and not run tasks manually via the management command and API.