Issue #1757566 by alexpott, aspilicious: Convert user account e-mail templates to configuration system.

PR for https://github.com/backdrop/backdrop-issues/issues/170
These config keys do not line up with the keys used elsewhere. For example "cancel_body" is "cancel_confirm_body" everywhere else in this PR. Let's keep the full string for each of these conversions, minus the "user_mail_" prefix since that's the name of the config file.