[DO NOT MERGE] Introducing AutoCompleteCascadeSelectField to ServiceCatalogMixin.

It is now possible to make 'cascading selects', i.e. narrowing the results of one autocomplete field according to the value selected in the other (parent/child relationship).

This concept is introduced here with ServiceCatalogMixin, i.e. the possible completions for device environment field are presented according to the value selected in the service catalog field.

[This pull request](https://github.com/quamilek/bob-ajax-selects/pull/15) (in bob-ajax-selects) have to be merged first (otherwise this one won't build)!