Generate user invitation emails

We need copy for the emails we'll be sending to users.

We might be able to get away with a single email but it feels like we probably want separate emails for the three groups of users

* Regular Reader users
* Active users who no longer use the Reader
* Inactive users who no longer use the Reader

Once we've identified the languages we'll be using, the email(s) will need to be sent for translation.

The issue names no files, tests, or entry points. First confirm the invitation-email workflow and the supported languages, then decide whether one message or separate copy is needed for Regular Reader, active former users, and inactive former users. Done means approved email copy for the required groups and a clear handoff for translation.

Written by the indexing model from the issue text.