If tree imports successfully, add supporting file.

This copies the input text (whether an uploaded file or pasted into a
form field) to a new file in the supporting-files area for this study,
so that it can be reviewed, downloaded, and deleted from within the
curation app. Fixes #418.

NOTE that this is in addition to the saving of input data to peyotl's
scratch folders. The latter is for diagnostic purposes.
This should probably be:

    size = os.path.getsize(os.path.join(*path_list))

to be on the safe side. What is here works as long as the standard lib shutil includes the line "import os". That will probably continue to be true, but I don't think there is a commitment to that in the shutil docs