Ability to configure email domain for notifications sent from Builder applications

All notification emails the app sends (approval requests, escalations, setup handoffs) currently come from no-reply@aha.io. Email providers tag every one of these with the "WARNING - EXTERNAL EMAIL" banner, and we're afraid employees will ignore or delete time-sensitive approval requests because of it.

We want to route these emails through a verified internal domain instead, so they arrive from the email address we currently use for these kind of notifications. According to Elle, the platform's built-in email sender can't be reconfigured, so we'd need to send through an external email service.