Add API for Customization content & add CLI checks

[Luzifer]

In order to have the same validation as in the frontend implementation the CLI needs to be able to know about the customization settings (accepted file types, file size, …) and check the local parameters match the server configuration.

• [x] Add API to expose the application configuration (JSON export of "customization")
• [x] In CLI do a pre-request to fetch the application settings
• [x] Let CLI check for accepted file types
• [x] Let CLI check for maximum file size