move xform to project

To assign a form to a project we only need the form's `formid`, the check is done [here](https://github.com/onaio/onadata/pull/333/files#diff-3d65ce77835a2f07194046eb827e8fe1R205). `publish_form` does the rest for checks if we are uploading an xlsform, `publish_form` does throw some exceptions if things are not as expected.