[Backport 2.1] Append shipment comment to shipment if appendComment is true

[JeroenVanLeusden]

Description

Add customer note to shipment if shipment is created with API and "appendComment" is set to true.

Fixed Issues (if relevant)

1. Shipment API won't append comment to email #11207: Shipment API won't append comment to email

Manual testing scenarios

1. Place an order via the website.
2. Create a shipment using the API with "appendComment": true.
3. Add comment to the shipment in same request.
4. Shipment email is being send with comment appended.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)