Allow groups to be migrated as new top-level groups

What does this MR do?

Related issue: #293952 (closed)

This MR allows groups to be imported as new top-level groups on instance. To achieve this on frontend following changes were made:

• new prop canCreateGroups is passed via data attributes
• dropdown list of the groups is generated with respect of new prop

Following changes were made on backend side:

• Relax validation on destination_namespace to allow empty string but not nil
• Handle empty destination_namespace in transformer and properly set parent_id to nil in this case

Video

New_Group___GitLab__4_

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)
• Select No parent from dropdown as target
• Click Import
• Ensure new group is imported and available (it should be empty intentionally)

Does this MR meet the acceptance criteria?

Please note, this MR is independent from !52796 (merged) - fixing status issue is solely purpose of !52796 (merged)

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