Make displaying "Message Count" widget a user setting in 3.2

## What?
When I start a new search in Graylog I want to make sure I use as much screen space as possible for message results and aggregation widgets.
Since my first time using Graylog a long while back until now I never found the Message Count any useful and rather a waste of space on the screen.

It would be great to have this as a user setting to disable "Message Count" from being displayed.

## Why?
With a user setting to disable "Message Count" it would make Graylog usage more efficient by preventing to delete (and confirm) the "Message Count" widget every time and leave it to those users who want it.
For anyone else allow saving the "wasted" screen space. (with "wasted" I don't mean to judge or complain - I'm simply lacking a better word)

## Your Environment
* Graylog Version: 3.2