T94 email on export -- fixes #94

[dolsysmith]

Email address is a required & validated field for submitting a request for dataset extracts. Email address is saved to the dataset_params.json file. The Celery task sends an email on completion of the requested extracts.

Note that it's necessary to modify the worker section of the docker-compose.yml file on the primary node. (Duplication of environment variables from the server section ensures that the former are available to the Celery worker for use with flask-mail.)

Successfully tested with up to 50 simultaneous jobs. Simultaneous jobs that use the same dataset_name seem to trigger SMTP errors, but that situation seems unlikely to occur in practice. (Simultaneous jobs with different dataset_names go through as expected.)

I'm still not sure why sfm_no_reply@email.gwu.edu wasn't working yesterday. We may want to monitor that closely in production to make sure users are getting notified.

For testing:

• Create a new dataset extract.
• Modify an existing extract and submit again.
• Submit a few longer-running dataset extracts in parallel.

[lwrubel]

This looks good to squash and merge now.