Mass user email script using Amazon SES

[LMNTL]

Checklist

1. If you've added code that should be tested, add tests
2. If you've changed APIs, update (or create!) the documentation
3. Ensure the tests pass
4. Make sure that your code lints and that you've followed our coding style
5. Write a description of your work suitable for publishing on our forum
6. Mention any related issues in this repository (as #ISSUE) and in other repositories (as kobotoolbox/other#ISSUE)
7. Open an issue in the docs if there are UI/UX changes

Description

Added a script to send broadcast emails to users using Amazon SES.

Notes

To test this, you'll need the environment variables AWS_SECRET_ACCESS_KEY, AWS_ACCESS_KEY_ID, and either AWS_S3_REGION_NAME or AWS_SES_REGION_NAME to be set.

Before running the script, make sure the constants at the top of the file are set. You'll need to create a blank HTML file in /scripts/ and set EMAIL_HTML_FILENAME to its file name, then do the same with a blank .txt file and EMAIL_TEXT_FILENAME.

You can run the script in test mode (doesn't send any emails) using:
./manage.py runscript mass_user_emails --script-args test

To force re-sending emails to users that have already been emailed:
./manage.py runscript mass_user_emails --script-args force

[jnm]

I have to admit, this took me an embarrassing amount of time to wrap my head around. I don't know if it's worth changing such well-tested code, but I'd offer for future consideration something like

string_sends = remaining_sends if remaining_sends > 0 else "unlimited"