New user welcome email

[rjw57]

A member who is newly added to the database should be sent a welcome email.

We should make use of the Django email sending infrastructure since that will automatically log any sent emails and use the correct "from" address, etc.

It's an open question on whether this should happen when a new member is created (i.e. a model "action") or as a batch process. Maybe having a "Notificiations" model which records when a given member has been notified?

My preference is for the latter and have the "send notification emails" be a separate Django application with an associated management command.