Migrate Group Avatar for each Group

Problem to solve

For the Group Migration to be a viable solution for the users migrating Groups, we need to achieve parity with the Group Export/Import feature.

Proposed Solution

As part of the larger epic targeting parity with Group Export/Import, this issue adds the Group Avatar to Group Migration. The Group Avatars in the destination group (and subgroups) should match the avatars in the source group (and subgroups).

Documentation

Following documentation may need to be updated:

• User docs: https://docs.gitlab.com/ee/user/group/import/
• Dev docs: https://docs.gitlab.com/ee/development/bulk_import.html

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