Project templates: Show if a group is selected for project templates on Overview

Problem to solve

With 11.2 and 11.3 (ideally) we deliver instance-level and group-level templates. Admins/Group owners can select a group to serve as project template source.
For Admins/Group owners, there should be an indicator on group Overview pages that this group is assigned as template source.

See conversation https://gitlab.com/gitlab-org/gitlab-ee/issues/6862#note_86890962.

Proposal

Let's add a UI indicator for Admins/Group owners when navigating on a Group that serves as project template source on instance and group-level.

What does success look like, and how can we measure that?

(If no way to measure success, link to an issue that will implement a way to measure this)

Links / references

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