Permission assignments bug fix

[noliveleger]

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 description of your work suitable for publishing on our forum
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

Save permission assignments correctly when usernames start with the same letter but contain non alphanumeric characters (such as _) and have a mix of upper and lower case.

Related issues

Fixes #3864

[bufke]

This introduces 2 additional DB queries on the bulk endpoint. Not blocking. If you know why, it would be nice to address if it was possible. I didn't dig too much into it.