Changes `Banzai::CrossProjectReference#parent_from_ref` to return the
project in the context if the project's `full_path` matches the ref
we're looking for, as it makes no sense to go to the database to find a
Project we already have loaded.
Updates notes-related services and rendering so this code can be
easily used for group-scoped resources (specifically Epics).
Related to gitlab-ee!5205
Thong Kuah