Add option to batch emails generated by user reports

[HelenWDTK]

At the moment, we receive an email each time a request is reported or classified as having a delivery error. This can sometimes result in hundreds of support emails being sent per pro batch. In order to reduce admin overwhelm, it would be good to have the option to receive a digest email once a day for report types that do not require an immediate response eg delivery errors, requests to add authorities or contact email updates.

Delivery errors alone generated 950+ emails over the last 16 months of running WhatDoTheyKnow.

Relevant to #7055