Deleting garden doesn't delete associated sensors

[tzou2024]

As part of the delete route for a garden, go back in and delete every sensor that has that deleted id in it's garden_id field.

[tzou2024]

This will probably be helpful:

https://motor.readthedocs.io/en/stable/api-asyncio/cursors.html

[jonaskaz]

We might actually never want to delete a garden or sensors. If we have sensor readings and we delete the sensor/garden that it references, the reading becomes meaningless (the reading will have an id that points nowhere). This could cause some of our future queries to fail unexpectedly if we are trying to link a reading back to a garden for example.