Prioritized fields

## What?

There should be a way to bring focus to (i.e. emphasize and make easy to identify) fields that are most likely to be important to someone looking at a given Message or the list of fields in the Fields side pane. Offhand I'm thinking of the prioritized fields at the top, with a "show remaining" expandable for everything else. For message tables, at least, perhaps that priority is determined in its configuration.

## Why?

There may be _many_ fields on a Message or in the Fields side pane's list of fields, making it difficult to filter through the noise. Much of the time we're only interested in drilling down on just a few of those fields, so that process should be made trivial.

## Your Environment

* Graylog Version: 3.3.2-1