Horizontal scrollbar not always visible in messages section

## Expected Behavior

Horizontal schrollbar in the messages column should always be presistent.
Horizontal scrolling by holding shift and use mousewheel already works, but I think not all people know this feature and for those who do, it would be nice to have a presistent scrollbar to see where you are.

## Current Behavior

If I tick as many or even all field boxes in the left column that horizontal scrolling is necessary in the messages column the scrollbar only shows up if I scroll down to the last message.

## Possible Solution

## Steps to Reproduce (for bugs)

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## Context

I made a input for our Veeam Backup software via SNMP plugin and the fields I got were kind of cryptic so I simply ticked all fields to see all columns in the message column.
At first glance I didn't find a possibility to scroll horizontaly wich confused me a bit.

## Your Environment

* Graylog Version: 2.4.3
* Elasticsearch Version: 5.6.5
* MongoDB Version: 3.2.11
* Operating System: Windows 7 / Graylog Running on Debian Stetch
* Browser version: Chrome