Asset snapshot cleanup task

[jamesrkiger]

Checklist

1. If you've added code that should be tested, add tests
2. If you've changed APIs, update (or create!) the documentation
3. Ensure the tests pass
4. Make sure that your code lints and that you've followed our coding style
5. Write a title and, if necessary, a description of your work suitable for publishing in our release notes
6. Mention any related issues in this repository (as #ISSUE) and in other repositories (as kobotoolbox/other#ISSUE)
7. Open an issue in the docs if there are UI/UX changes

Description

Adds regular asset snapshot cleanup maintenance task. Reworks existing test to make sure latest versioned snapshots are preserved.

Notes

I would appreciate feedback on organization here. KPI already has a tasks.py file for celery tasks, and the new remove_old_snapshots() is not a celery task, so I didn't want to put it there. Currently it is in a new maintenance_tasks.py file. I was thinking of putting them both in a tasks folder. One could be celery_tasks.py and the other could stay as maintenance_tasks.py.

Down the line, I'm also wondering if I should be removing the existing management command code (here for snapshots and here for the DeleteBaseCommand class) once I replace the other task (removing old import tasks) that relied on it.

Related issues

Partially replaces work in #2434

[bufke]

I'm also wondering if I should be removing the existing management command code (here for snapshots and here for the DeleteBaseCommand class) once I replace the other task (removing old import tasks) that relied on it.

Yeah for sure. Either now or later. We may want to consider our alternative abstractions like RemoveBaseCommand but not until we have more code to actually abstract.

[bufke]

Approved pending consideration of my comments.