Pipeline Source Editor: Poorly arranged buttons on a small screen

## Expected Behavior
It should always be clear where buttons are belonging to and where not.

## Current Behavior
On a small screen the "Update" and "Cancel" buttons seem to connected to the reference box:

## Possible Solution
Needs some CSS ❤️

## Steps to Reproduce (for bugs)
1. Go to a Pipeline Rule Source Editor Page
2. Make the window small

## Your Environment
* Graylog Version: 5.1