Stream response on endpoint /<username>/xformsManifest/<form_id> (#…

…2493)

* stream response on endpoint /<username>/xformsManifest/<form_id>

stream response to avoid serialization using too much memory

* wrap respsonse in brackets

* separate streamed data using comma

* specify content_type when streaming response

* update stream response for /<username/xformsManifest/<form_id>

stream XML instead of JSON

* use default queryset_iterator chunk_size

* adjust failing tests

* suppress lint error line-too-long

* fix lint error

fix pylint: redefined-builtin / Redefining built-in 'hash'

* suppress lint rule line-too-long

* remove unncessary kwarg

* refactor code

* add docstring

* fix docstring typo

* refactor code

* address failing tests

* add method docstring