uploaded media check / cleanup

[rlskoeser]

We have a sizable amount of data in our upload folder, would be good to have a way to prune content that is no longer used/needed (or was uploaded and never used).

Wagtail images have a way to check where they are used - see wagtail/bakerydemo#114 (slightly out of date) and wagtail docs on reference groups. Note that this doesn't report on images used within streamfields, although I think we generally don't allow that.

We would also need to query for other objects that link to images directly (projects, profile pages, events) since I don't think they're set up to count as a wagtail reference.

Maybe a manage command that could report on and optionally clean up unused images.