Validate group names before importing

What does this MR do?

Add validations when importing groups:

• Sync validation for correct name (shared regex with /groups/new)
• Async validation on existence of group on target instance

Related issue: #300427 (closed)

Video

New_Group___GitLab__9_

added: Please ignore a weird blank area at the top of the page, it's issue of Screencastify plugin used for recording

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)

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