Allow member mapping to map importer user on Group/Project Import

What does this MR do?

Related to: #212298 (closed)

Enable Gitlab::ImportExport::MembersMapper to map the importer user if this user were part of the exported Project/Group.

Currently, when importing a project or group from a GitLab export, we try to map the exported members of the project/group on the target GitLab instance. The mapping, though, is not taking the importer user in consideration even if the importer user was a member of the exported project/group.

This MR changes this behavior, to include user doing the import on the member mapping when importing a project/group.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
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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
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