Include most recent date in page not found and access denied reports

Problem/Motivation

The reports at /admin/reports/page-not-found and /admin/reports/access-denied would be more useful if they included the date and time of the most recent instance of each item.

Steps to reproduce

1. Enable dblog
2. Record some 404 or 403 errors
3. View the Top 'page not found' errors or Top 'access denied' errors report

Proposed resolution

Add a Latest instance column

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet