Add maintenance function to remove old import tasks

[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 maintenance task for cleaning up old import tasks.

Notes

I created a new test file for this. All other import task related tests are run in API test files and this didn't seem relevant to API testing.

Related issues

implements work from #2434

[bufke]

LGTM pending the review of my comments