When assets have been copied with rake admin:copy_assets, then rails_admin should NOT insert
another ActionDispatch::Static middleware, because it eclipses Rails's native middleware and assets
are loaded from rails_admin's public folder instead of the application's public folder.
Fixed by checking if any of the rails_admin subdirectories exist in any of the static asset classes
(images, javascripts, stylesheets).
Also, if the middleware is inserted, it should be done AFTER, not IN FRONT of Rails's own static asset
middleware, to permit precedence to the application's public folder over the gem's.

…g, but didn't check for the files to be present,

which is cool.

Model name in table is now using label override if present.

@page_name should use overridden model name.
Same for breadcrumbs.
Spec coverage added.

where @model_config.list.label was called, but @model_config.navigation.label should
have been called.