Show GitLab Migration as option when importing project

Problem

At the moment there GitLab Migration is not showed as an option when going to New project -> Import project, as migrating only projects in not possible yet. However it would be good to inform user about an option to migrate projects within a group with GitLab Migration.

Proposed solution

Add an affordance on New project -> Import project- "Are you trying to migrate projects from GitLab? You can move projects under a top-level group with GitLab Migration". Add a link to docs and link to New group -> Import group -> GitLab Migration.