Column Pivots should optionally include empty columns

## What?
In a data table with multiple column/row defined, the column/row is omitted when the value is empty. See picture for better explanation. In this example there was non errors (Succeded = False) for the live environment, nevertheless, there should be an option to show the column. The logic should be, if some of the other "groups" contain the column/row, all "groups" should contain it.

The option should be configurable either on a per column/row basis (by clicking on the blue value item; same pop-up like to configure the "Number of values") or at a general basis (by clicking on the wrench icon; same pop-up like to configure the rollup option).


## Why?
* Readability / more beautiful data visualization (perhaps not for the techie, but for the manager)
* Some people may think a missing column was forgot, instead of there is no value

## Your Environment

* Graylog Version: 3.3.5
* Elasticsearch Version: N/A
* MongoDB Version: N/A
* Operating System: N/A
* Browser version: N/A