Perf: Optimize member list on the landing page for collective with large number of members

[Koc]

📝 Summary

This is continuation of #2443. I had an attempt to implement lightweight API endpoint in circles module but stuck with it due to federated users. This is a really loong path which I'd prefer to avoid. Also keep in mind that we're reloading this members list every 1 minute due to background fetcher and endpoint latency goes up to 10s in our organization.

So, instead of that we're loading only 15 users for a landing page. And then re-loading all users once we toggle editor mode from readonly to edit. This improves performance of BE and do not degradates users autocomplete functionality in the editor.

🏁 Checklist

• Code is properly formatted (npm run lint / npm run stylelint / composer run cs:check)
• Sign-off message is added to all commits
• Tests (unit, integration and/or end-to-end) passing and the changes are covered with tests
• Documentation (README or documentation) has been updated or is not required

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI tools
• The AI-generated content was reviewed, comprehended and tested by a human