Migrate Group Import Form to GitLab UI

What does this MR do and why?

Also make a gitlab_ui_form_with helper styled similarly to the gitlab_ui_form_for helper, as there are a few instances of form_with in our codebase.

Eventually, we should also add a gitlab_ui_form_tag helper, but that can be later.

Screenshots or screen recordings

How to set up and validate locally

1. Go to create a group
2. select "import group"

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.

For #374832 (closed)