[feature] Configure notification body via API

**Version**
latest

**Feature request**
It would be very useful to be able to configure the notification body through the API. This way we can fully automate our deployment. Otherwise we have to manually configure it using the web form.

**Desired solution**
- A new API endpoint to configure the system wide notification body.

Start by locating the existing API routes and the web form that currently configures the system-wide notification body. Trace how that setting is stored and used for notifications, then add the requested API capability. Done means an automated deployment can configure the notification body without using the web form, with the relevant API behavior verified.

Written by the indexing model from the issue text.