Default `Cron Expression` value when creating a new event definition

## What?

There should be a default `Cron Expression` value (e.g. `0 */5 * * * *`) when creating a new event definition.

## Why?

Makes it a bit clearer what the expected format is (mainly: number of elements), and saves users a little time.

## Your Environment

* Graylog Version: `6.2.2`