added DDRLocalFile.from_json() function

Finally you will no longer have to load an entity just to load
a File...
first stab at entity/file-to-CSV exporter
mv "importers" -> "migration"; added export to CSV

The "importers" module now contains functions for exporting
entities and files to CSV.

- Changed module name to reflect additional functions.
- Use same options for both CSV reader and writer.
- importer code is unchanged.
- Walkthrough for new export code.
- Finally added a File.from_json() function.
mv add_file(),add_access() from tasks to models.DDRLocalEntity

This has been a long time coming.  Back in the mists of time there
was a circular dependency that prevented me from putting these
functions in models where they should have been all along.
completed migrations.densho.export_(entities,files) functions

- Increased CSV maximum field size to accomodate some files' XMP.
- Single function for generating CSV filenames.
- Encode CSV data as UTF-8.
- Return CSV filename if successful.
added Export Objects/Files to CSV

- Buttons on collection-detail page.
- CSV file generation is done in Celery task.
- Files are downloadable via URL as long as /tmp/ has not been
  emptied.
fixed python noob indentation mistake. stoopid.
make sure tmpdir exists before trying to export
force non-string values to str so we can run utf-8 encode
better implementations of mktmpdir and force-to-string
Merge branch '155-export'

Conflicts:
	ddrlocal/webui/tasks.py