Rails 5 changed the initialization order, and now our initializers were
running before the `append_assets_path` initializer for each engine,
which prepended application assets to the custom assets we prepended in
the initializer.
Moving the code to the `config.after_initialize` code didn't work
either, since the paths added there were ignored by the application.
Adding another initializer to the Rails Engine is a hack, but solves the
problem.
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