UX: Improve route hierarchy in for user-invites (PR #14583)

Previously the sidebar was being rendered in the -show routes, which meant that it disappeared and re-appeared when each tab was loading. This commit creates a parent user-invited route with the sidebar, and then renders the -show view in an outlet.

To avoid an extra HTTP request, the invite counts for the sidebar are fetched by the -show routes, and then applied to the parent controller. This means that there can be a very slight delay before the counts are displayed, but it is almost unnoticeable in normal use.