UK|ME: test helper to make submission with attachment
UK|ME: add AttachmentSerializer
UK|ME: add AttachmentViewSet
UK|ME: add AttachmetObjectPermission class
UK|ME: add AttachmentFilter
UK|ME: apply permission and filter to AttachmentViewSet
UK|ME: use nested attributes in HyperlinkedMultiRelatedField, ensure we return proper data url
: add url field to attachment endpoint
: updated serializer fields
: code refactor, move renderer to renderer module
: enable file download when file extension is supplied
: test for download_url
ME|UK: added download url to attachment serializer
ME|UK: added instance filter functionality and test for the instance filter
ME: added media api url
ME: added documentation for Attachment viewset
ME: code refactor on some file
PLD: make comments flake8
Merge pull request #496 from onaio/318-attachment-api-endpoint

318 attachment api endpoint