Group migration - Import one group - frontend

# 🚧 Blocked by merge request 🚧 !49699 (merged)

Target branch will be set to master after this MR land

What does this MR do?

Related issue #249160 (closed) This MR actually invokes import group from remote gitlab instance

Video

New_Group___GitLab__1_

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
6. Click Import
7. When status will be changed to done you should see group "imported" to your instance (currently only epics and subgroups are imported, see &4374 (closed) for details)

Screenshots (strongly suggested)

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done