Frontend for showing Projects under Groups tree

What does this MR do?

Adds frontend for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13017

Why was this MR needed?

Makes tree view show Projects on Groups Dashboard and reuse same tree for Subgroups tab under Groups homepage

Screenshots

Groups Dashboard

Default View (everything collapsed)

Expanded View (Groups first, then Projects)

Expanded View (Has More Groups link)

Subgroups under Group Homepage

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • [ ] Has been reviewed by Backend
  • [ ] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

#30343 (closed)