Race condition on `ProcessedImage`'s TaskEventMixin with ManyToManyField

[banesullivan]

There's a flaky error happening when POSTing a ProcessedImage where the save event that triggers the task event is called, seemingly before the ManyToManyField is populated.

To my understanding, you have to save the model before populating the ManyToManyField and thus a race condition of sorts happens where the task checks for the needed source image in _processed_image_helper(). This is flaky as the refresh_from_db() call sometimes gets it in time, but sometimes does not:

https://github.com/ResonantGeoData/ResonantGeoData/blob/78065e2d44c27e2715a47028a33c3eae89ebca71/django-rgd-imagery/rgd_imagery/tasks/subsample.py#L27

[banesullivan]

I wonder if we need to change the signal to be m2m_changed instead of post_save?

[banesullivan]

AND if the group field is updated

[banesullivan]

I need this working by tomorrow, so I added fb26d3c to work-around it for now

No longer a priority .... this hack works well enough