Group migration - Show available groups - frontend

# 🚧 Blocked by merge requests 🚧

!48772 (merged) !48784 (merged) !49697 (merged)

Target branch will be set to master after these MRs will land

What does this MR do?

This MR adds displaying of available groups for bulk import

Video

New_Group___GitLab

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. Clicking Import will change status to Scheduling but nothing more, it is handled in #249160 (closed)

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #249157 (closed)