Move attachments field to raw and markdown cells
Move attachments field to raw and markdown cells

Previously, the `attachments` field was specified in the metadata of raw and code cells. Corrected to follow the specifications in the docs: http://nbformat.readthedocs.io/en/latest/format_description.html#cell-attachments
jupyterlab/services,jupyterlab/services,jupyter/jupyter-js-services,jupyter/jupyter-js-services,jupyter/jupyter-js-services,jupyterlab/services,jupyterlab/services,jupyter/jupyter-js-services