[Feature] Add configurable email notification delay

I have searched the existing feature requests to make sure this is not a duplicate request.

• Yes

The feature

Currently, email notifications for album updates are sent every 5 minutes. When a picture is added before the time is completed, it is reset and the time starts again (this is my understanding from looking at the PR that added the delay).

It would be really useful to have the ability to configure this delay, either by specifying the number of minutes or by providing a CRON schedule either through an environment variable or the administration page.

Here's where the current 5 minute delay is configured:

Platform

• Server
• Web
• Mobile