Properly translate renamed resources

This is for #1884, and is a re-wiring of #1885.

The initial problem that #1885 solved was that if you used the `:as` option, translation just didn't happen:
```ruby
def resource_label
  if @options[:as]
    # just return the string
  else
    # actually do translation
  end
end
```

Unfortunately, to do so #1885 added a lot of duplicated code. This PR is an attempt to find a middle ground.

I had to create my own `translate` method, because for some reason `String#human` wasn't properly respecting `i18n_key`
 and the `activerecord.models` I18n scope.

But the result is that now you can rename a resource:
```ruby
ActiveAdmin.register Foo, as: 'Bar'
```

and can have custom translations for `Bar`
```yaml
en:
  activerecord:
    models:
      bar:
        one: "Baz"
        other: "All the Bazs!"
```