List associated notification names in detail section of event definition

## What?

When looking at the list of event definitions on the Event Definitions page (`/alert/definitions`) you'll see a message similar to this under each:

> Runs every 1 minutes, searching within the last 1 minute. Triggers 1 Notification

If you click `Show details` next to that message you'll see something like the following:
* `Status: runnable`
* `Last execution: 2021-08-25 13:24:38.823`
* `Next execution: 2021-08-25 13:25:33.648`
* `Next timerange: 2021-08-25 13:24:33.649 --> 2021-08-25 13:25:33.648`

These could be considered related to the first part of the line above, but to see notification details in the UI you need to click one of the event definition links (`/alerts/definitions/ID` or `/alerts/definitions/ID/edit`).

This is a request to include a list of Notification names (perhaps as links) in the expandable "details" section of event definitions listed on the Event Definitions page.

## Why?

1. Convenience when needing to determine where multiple alerts went (or will go)
2. Having `Triggers # Notifications` right next to `Show details` suggests that the details will include some sort of notification information

## Your Environment

* Graylog Version: 4.1.3