Scope related route names in a similar manner to relationship routes

[lgebhardt]

This dodges an issue where the related route names might conflict with
index routes (e.g. a List that has_many :items, class_name: 'ListItem'
would have previously used list_items for the related resources and
conflict with the list_items index route for the ListItem class`)

cherry picked from 886cbe5 in release-09

Closes #1251

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