Move excel temp to S3 and add toggle to enable sending event emails

* Previously, Excel reports were stored in a local file in the filesystem. This breaks multi-instance apps. This patch moves the temp files to S3. I recommend setting a lifecycle policy on the S3 bucket to automatically expire this content after a period of time.
* Previously, all events sent an email to the attendee when the attendance status changed (e.g., they RSVP'd), but there was no way to disable this functionality. This patch adds a boolean field to configure this on a per-event basis, and adds an override to the Visit model so that event emails are _never_ sent for High School Visits. To maintain existing functionality in existing apps, though, the migration will set this boolean to +true+ for all previous events. The default moving forward is +false+.