Group migration - Show available groups - frontend

As part of the GitLab Group Migration MVC epic, this issue will add the ability to show a list of top-level groups available for import, as discussed in this comment.

The list will contain the source group path/name, (editable) destination group path/name, status of the import, and the import button.

Out of scope

Solving pagination/infinite scrolling for long lists of groups is not in the scope of this issue.

Designs

Designs and prototype posted in the issue UX for GitLab Group Migration - MVC #237925 (closed)

This page may contain information related to upcoming products, features and functionality. It is important to note that the information presented is for informational purposes only, so please do not rely on the information for purchasing or planning purposes. Just like with all projects, the items mentioned on the page are subject to change or delay, and the development, release, and timing of any products, features, or functionality remain at the sole discretion of GitLab Inc.