Will/external submissions app

* Install `submissions` from an external source: https://github.com/edx/edx-submissions
* Move `openassessment` up one directory level and delete the `apps` directory.  This has the pleasant side-effect of fixing test failures that occur when running `./scripts test openassessment.assessment`

When we merge this and release it to edx-platform, we should add `edx-submissions` to the edx-platform requirements.  I've chosen to use PyPi rather than installing the Git repo directly so we'll get errors if the version in platform doesn't match the version in ora2.

@ormsbee @singingwolfboy