Optionally log the sending of e-mails (ENV variable)

[orenyk]

We should log the sending of all automatically sent (e.g. rake task sent) e-mails using Rails.logger to help debug future e-mail issues (e.g. #1125), but on larger instances this may prove to be prohibitive. Therefore, we should also introduce a configuration setting (via an ENV variable) that determines whether or not to log e-mails.

[orenyk]

For posterity - we're probably going to use ActionMailer Observers for this to keep things nice and DRY. See #1316 for discussion.