Search box for "add annotators" is case sensitive

[ianroberts]

Describe the bug

Most of the search boxes including the project search page and the user management page do case-insensitive search, but the list of annotator usernames you get when adding annotators to a project is case sensitive.

To Reproduce

Steps to reproduce the behavior:

1. Create a user named "Example"
2. Create a project
3. "add annotators" to the project
4. Search for "exam" in the user list

Expected behavior

The "Example" user should match the query

Additional context

Most of the other search boxes were made case-insensitive in PRs #406 and #407, but this one was missed at that time.