I removed the caching of `managed_enterprises` in Permissions because
it's just a scope and calling it again is very cheap. And that makes the
method a lot easier to read now that we have a conditional here.
Accessing the managed enterprises via the user instead of a separate
scope on the Enterprise model also reduce the SQL queries. We may want
to use this method in more places. I prefer to keep the
admin-conditional in a permissions class instead of in the model.
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