Enable unmapping of objects in response

This is a potential fix for story: https://www.pivotaltracker.com/story/show/61957752, although Kostya wasn't sure if mappings were disabled intentionally.

I was a bit confused about the `allow_mapping` property as my first thought was to correct it in the response child mappings (https://github.com/reciprocity/ggrc-core/blob/develop/src/ggrc/assets/javascripts/pbc/response.js#L101), but it's already set to true here. It seems to get overwritten by the audit one and I'm not sure whether this is a bug or intended behavior?

PS: From what I can tell changing `allow_mapping` to `true` here has little effect as the default for `allow_creating` is `true` and audit uses `allow_mapping_or_creating` in all instances that I was able to find.