Log a message whenever an email is sent

[seanh]

Log a thoughtfully-designed message whenever an email is sent. This will enable us to use the logs (in Papertrail) as one way to monitor (and set alerts for) our email-based features.

The log message should not include any potentially sensitive information like: usernames or userids, email addresses, the contents of annotations, or the subject lines or bodies of emails.

The log message should include helpful information like the type of email being sent (identified by the tag, see: #9286), the user.id of the sender (when an email has a "sender": for example in the case of a reply-notification email the sender would be the creator of the reply annotation), the user.id of the recipient, the annotation.id when applicable (for example reply-notification and mention-notification emails have an annotation), etc.

Since the details to be logged will vary depending on the type of email, the generic email send() method should probably have a log_data dict where the caller can supply custom data to be appended to the log message.