PlantUML diagrams require unauthenticated and publicly accessible PlantUML service

When integrating with PlantUML, the resulting diagrams aren't being served from GitLab, but the PlantUML service itself. This means that the PlantUML service must be publicly accessible, without any form of authentication. If not, the user sees broken images.

Possible solutions:

• Detect client side whether PlantUML is accessible and show a message instead of broken images.
• Proxy the diagram from the service through GitLab.
• Save/cache the diagram in GitLab after generating.

The last option would have the added bonus of the image being shown even if the PlantUML service is unavailable.