CE backport of changes that introduce custom instance-level templates to EE

What does this MR do?

Ports non-EE-specific code in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7000/commits to CE

In particular, I introduce a ::TemplateFinder to provide a good injection point to add the custom templates to the vendored templates that CE has access to.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

Merge request reports

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