Fix breadcrumb links for namespaced parent models

[seanlinsley]

The previous code broke nested resource (e.g. /users/1/stories) had a
parent class that was namespaced inside a module.

# Given these models
class Foo::Users   # has_many stories
class Foo::Stories # belongs_to user

# and this AA routing for stories
ActiveAdmin.register Foo::Stories do
  belongs_to :user, parent_class: Foo::User
end

# you'd get this error
"undefined method `find_by_id' for Foo:Module"

This commit resolves the problem by directly asking the AA config
what the parent class is, instead of trying to guess based on the URL.

Fixes #2051