Use streaming responses

See activeadmin/activeadmin#2689
> The unused in-memory DOM representation more than doubles the memory usage required to service a request and complicates the implementation to having a seperate buffer per element. I think a lot can be gained my dropping the in memory DOM representation: a single ActionView buffer can be passed from parent to child, lowering memory usage and allow us to use [streaming responses](http://api.rubyonrails.org/classes/ActionController/Streaming.html).