fixes #5753: Engines can now override autocomplete path used in FiltersHelper

As you noted Foreman uses ```/resource/auto_complete_search``` format for auto-complete urls (as opposed to parametrized urls), which is what I had in mind when writing this. The issue you mention in the latter comment arises when override isn't being used -- by default FiltersHelper assumes that a resource path exists, which may not be the case when you rely on angular to route your requests.

Having said that, I just added support for passing in resource name in the override block. I still think it's a good idea to stick to foreman's format of urls in general, but I realize that you may need to sometimes tailor search paths to specific models...