Set a column as table index in mail_safety_dashboard

Problem/Motivation

Currently, the mail_safety_dashboard table does not have an index column -- https://dev.mysql.com/doc/refman/8.0/en/mysql-indexes.html

As the table can balloon to be very big, should either the sent or mail_id column be set as an index?

Proposed resolution

Add an update hook to set a column to be an index