Don't sort output config fields by optionality

## Expected Behavior

The config fields of my output plugin appear in the order I registered them.

## Current Behavior

If some of them are optional, they are shown below all others.

## Possible Solution

Don't [sort the fields by optionality](https://github.com/Graylog2/graylog2-server/blob/eec520f92acfe69ed6bc86b1bb259bfe07cc59de/graylog2-web-interface/src/components/configurationforms/ConfigurationForm.jsx#L156).

## Steps to Reproduce (for bugs)

Register two output config fields and make the first optional.

## Context

https://github.com/Graylog2/graylog2-server/pull/4746#issuecomment-384253076

## Your Environment

(not relevant)