What does this MR do and why?

Migrate to form alerts to Pajamas

Related to #364953 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Run your GitLab instance as SaaS (use GITLAB_SIMULATE_SASS=1)
2. Open http://localhost:3000/users/sign_up/groups/new
3. Choose the same name for the group as one of already existing
4. Observe the errors

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.