Improve visual representation of JSON fields

Hi, I'm working with some unstructured JSON data so writing meaningful extractors/pipelines is not really feasible.

## Expected Behavior
I'd like the message interface to format JSON fields in a nice readable way using indentation for nested objects and so on.

## Current Behavior
Currently, JSON fields are formatted as unreadable plain strings (data is not real, just for the showcase):


## Possible Solution
Add an extractor and/or pipeline function which would pretty-print a JSON field. Just pretty-print, without changing the format or names so it is possible to copy it if needed.

## Context
I'd like to use Graylog to monitor events happening in the system, most of those carry some big unstructured JSON payload which I would like to inspect manually.

## Your Environment
I've tried both Graylog 2.5 and 3.0

* Graylog Version: 3.0
* Elasticsearch Version: 6.5.4
* MongoDB Version: 3
* Operating System: Linux
* Browser version: Chromium 71