Reminder notification/email for Approvals

Description

Our team has only about 4 developers but we work varying schedules. We make full use of Approvals for MRs but as some have mentioned it's difficult to stay on top of reviews a developer needs to do as mentions in #769 (closed) and #765 (closed).

Proposal

My addition to the proposals above would be to add the possibility of configuring a nudge/reminder notification and/or email which is triggered periodically.

Some formats can be:

• Daily Digest - You get one aggregate email with all the MR awaiting your approval every morning around 7 AM your time.
• Threshold Days - You get an email/notification each day following X days without approving a MR. X being configurable.