Adaptation of appengine-mailer

This repo has been forked from the original appengine-mailer repo by Toby White.

See the original README for more detailed information.

Summary of changes from the upstream version:

• Adapt to 2nd generation App Engine runtime
• Adapt to Python 3
• Replace deprecated APIs with current ones
• Remove non-essential files and functionality
• Get the signature key from a GCP secret whose name is passed in the GMAIL_SECRET_NAME environment variable

Deployment

From the root of the repo

gcloud app deploy appengine_mailer/

will deploy to the current GCP project set by gcloud config set project.