Prior to this commit, we were treating all NavigationLink attributes as
unique. So, were we to change a position of one of the NavigationLinks
during the add_navigation_links rake task, we would have created a new
NavigationLink (and the only one would have remained).
With this commit, we're introducing the concept of the NavigationLink's
surrogate identity, that is to say if we have two NavigationLink objects
with the same `url` and `name` we should consider them the same
NavigationLink. This allows us to update properties of those
NavigationLinks (via the rake task) without the risk of creating new
entries.
This unblocks PR #16268 which addresses #16076.
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