Schema field order not saved

## Gentics Mesh Version, operating system, or hardware.
0.31.4 running in docker-compose https://github.com/gentics/mesh-compose

## Operating System
MacOS 10.14.3 (18D109)

## Problem
On creating or updating Schemas and Microschemas, the order of the ```fields```-Array is not saved.
Instead, the order is determined alphabetically referring to the ```name``` property string of a field.

## Expected Behavior
As specified for the frontend, the user can determine the order of Schema and Microschema fields.

Related to frontend issue: https://github.com/gentics/mesh-ui/issues/122

The issue names no files, tests, or entry points. Start by reproducing schema and microschema creation or update through the API, then trace where the fields array is persisted or serialized. Done means the submitted field order is retained rather than replaced by alphabetical order.

Written by the indexing model from the issue text.