Send SLO reminders for review resolution

[jrobbins]

This is progress toward implementing improved SLOs for both an initial response and a review resolution. Now that we can calculate when a review is due for a resolution, we will start sending email reminders about review resolutions that are due or overdue.

In this PR:

• Consider all gates that have active reviews and organize any due or overdue ones into four lists: initial response due, initial response overdue, resolution due, and resolution overdue.
• Generate four sets of email notifications based on those four lists.
• Make the wording of the reminder email vary based on initial response vs. resolution.
• Add unit tests and golden data files.

[jrobbins]

This relates to #4634.