Handle submission XML with namespace or encoding declaration

[jnm]

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

Previously, submissions with namespaces or encoding declarations in their XML could not be bulk edited or cloned.

Notes

Makes some progress toward standardized XML handing with defusedxml. In the future, all XML work should be done in kpi.utils.xml.

Related issues

Fixes #4283