Option to search inside alert description

At the moment, the search feature in the alert tab only searches inside the title of the event, it would be a good addition to allow the search field to look inside the Description field.

## Why?
To simplify the search process in an instance that has a bunch of alerts, not to now overwhelm the title with informations.

## What?
Adding a simple button that would enable search inside the Description field.

## Your Environment
* Graylog Version: 6.3.1 (But not working in 6.3.4)