Add default tester autocomplete modal

[M4dotsuki]

Summary

• add a modal-based bulk default tester picker that uses the existing typeahead.js + JSON-RPC stack
• call User.filter only for users with auth.view_user; users without that permission keep the existing prompt flow
• reset the modal input between opens and initialize typeahead only once
• support autocomplete lookups by username, email, or numeric user ID

Fixes #2510

Validation

• npx eslint testplans/static/testplans/js/get.js
• npx webpack

[atodorov]

If you are willing to work on this we need to resolve comments on older contributions first, then build on top of that, not starting everything from scratch.

Let me know if you are willing to follow up on it and I'll make some adjustments to unblock your work.

[atodorov]

This seems to duplicate of
#3250.

Can you explain what's the relationship of your code with the previous contribution outlined above ?