WIP: Import groups

What does this MR do?

Related issues: #249160 (closed) #249151 (closed) #249157 (closed)

This MR is previewof import groups design. Before getting merge it will be split in 5-6 separate merge request

Screenshots (strongly suggested)

Testing gudelines

1. Go to https://gitlab.com/-/profile/personal_access_tokens and create token with api permission. I suggest putting expiration date as tomorrow for security reasons. Copy token to clipboard
2. Login to https://gitlab-review-xanf-impor-27emvs.gitlab-review.app/ with login & password available in 1Password
3. Go to https://gitlab-review-xanf-impor-27emvs.gitlab-review.app/groups/new . Open Import group tab. Take a look at new tab design
4. Set https://gitlab.com as source url and use personal access token, generated on first step
5. Review status page (import button is not intended to work now)