UI improvement in Event Definitions step

The stepper form to create a new Event Definition has a small UI issue where the buttons to add a custom field are mounting on top of the step buttons (previous and next)



## Expected Behavior

Add space between the step buttons and the form steps

## Current Behavior

There is no space between the step buttons and each step of the form

## Possible Solution

Add top padding or margin to the div surrounding the step buttons

## Steps to Reproduce (for bugs)

1. Navigate to Alerts tab
2. Start a new Event definition
3. Observe on the Event Fields step, the buttons Add Custom Field, Done and Cancel seat on top of the Previous button

## Context

## Your Environment

* Graylog Version: 4.3.0
* Java Version: 8
* Elasticsearch Version: 7.10
* MongoDB Version: 5.0
* Operating System: Linux (Debian based distro)
* Browser version: FireFox 101.0.1 (64-bit)