Add a welcome email to the first user sync.
Merge branch 'master' into mm-welcome-email
Don't send welcome email if users is older than 48 hours.

Sort of a safety net in case, for some reason, synced_at should
remain empty for users.
Blank?
Use only configured address.

In the context where the messages are sent, the from address is
set to the full name and email address.
Add welcome email subject. Fix broken tests.
Track a metric when sending a welcome email.
Smaller font size for the footer.
Use logger.
Merge pull request #218 from travis-ci/mm-welcome-email

Add a welcome email