Automatically expand the size of "All Messages" widget reach the bottom of the screen

## What?
When running searches in Graylog 3.2 the first thing I nearly always do is to resize the "All Messages" widget (basically the main search results) so it reaches the bottom. Otherwise it would be just a huge amount of unused screen space.
This is a step that can be avoided by automatically adjusting the widget to fill the screen till the bottom.

## Why?
Improve efficient usage of Graylog 3.2 by not always having to manually resize the widget.
Roughly 50% of the screen space is blank initially when starting a new search (orange area in screenshot).

## Your Environment
* Graylog Version: 3.2