Group migration: maintain import status across reloads

What does this MR do?

Related issue: #321684 (closed)

Testing guidelines

• Enable feature bulk_import via Feature.enable(:bulk_import)
• Open "New group" (/groups/new)
• Use https://gitlab.com as source instance and GeK1Nis4j-SY1X4sqE5c as personal access token (this token is from separate user on GitLab instance with 0 real data available, so we do not expose any security risks here)
• Initiate import of any group
• While status is Running refresh page
• Ensure data is correctly preserved
• Wait for Done status
• Refresh page
• Ensure data is correctly preserved

Video

New_Group___GitLab__5_

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

No changelog, as feature is behind feature flag