Resolve "Group migration - Authenticate user - frontend"

What does this MR do?

This MR introduces new "Connect to another instance" UI

Related issues: #249151 (closed)

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. Enable feature flag by running Feature.enable(:bulk_import)
3. Go to /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. See status page (should be almost blank at this point, this is intended, handled in #249157 (closed)

Screenshots (strongly suggested)

:Expected design by UX team:

Known & approved differences between screenshot and implemented version:

• Buttons are not disabled, instead user can always press button and see what he/she is missing

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content