Add admin / config page to enable/disable PSA

Problem/Motivation

In #3046855: Notify site administrators when new PSA is posted we are adding the ability to email admins. Depending on when things land, we might want to add a UI config option to enable/disable the emailing. But definitely we want a kill switch for PSAs overall. So we don't annoy the random folks who want to disable that feature. Or, more likely, when that service lands in core and we want to disable it from config so we don't send duplicate messaging.

Proposed resolution

Add a kill switch to the service. Optionally (depending on the order of things if #3046855: Notify site administrators when new PSA is posted lands first, add a UI element to disable sending emails.

Remaining tasks

Do it.

User interface changes

API changes

Data model changes

Release notes snippet