Refactor empty state in `groups/components/app.vue` from HAML to Vue

Summary

Follow-up to !84397 (comment 971312865)

The empty state in groups/components/app.vue is shown by using DOM manipulation to unhide some HAML. This is kind of hacky and we should refactor these empty states to Vue so they are easier to manage and understand.

Improvements

• Reduce technical debit

Risks

Low risk, minor refactor

Involved components

• app/assets/javascripts/groups/components/app.vue#L215
• app/views/groups/_archived_projects.html.haml
• app/views/groups/_shared_projects.html.haml