Investigate mass email API solution

Currently to send emails we use Gmail via SMTP. This has become problematic on a number of fronts, as Gmail does not recommend this use case and has disabled mail accounts for spamming in the past, and sometimes the SMTP connection goes stale and the server has to be manually restarted to fix the connection.

We would like to find a more reliable API solution for sending mass emails going forward.

## Minimum Requirements
- Use via API, not SMTP
- Can send emails from democracylab.org domain

## Judging Criteria
- Cost
- Reliability
- Tools/Reputation for not triggering spam filters
- Existing python support

## Options to investigate
- Mailchimp (DemocracyLab already uses Mailchimp for our mailing lists)
- https://elements.heroku.com/addons#email-sms

Start by comparing Mailchimp and the email services listed in the Heroku add-ons directory against the minimum requirements and judging criteria. Confirm API support, sending from the democracylab.org domain, cost, reliability, spam-prevention reputation, and Python support. Done means documenting a justified service recommendation.

Written by the indexing model from the issue text.