PlantUML Support: Same behavior for live preview and read view

[schnatterer]

When clicking edit (split screen mode) the following PlantUML syntax leads to a rendered PlantUML diagram:

```uml
@startuml
Bob -> Alice : hello
@enduml
```

However, after saving (in read view) the following is rendered

If using the syntax without fenced code blocks:

@startuml
Bob -> Alice : hello
@enduml

it is the other way round: No rendered image in live preview but a properly rendered image in read view.

The most intuitive solution from a user perspective would be to support both syntaxes in both views, i.e.

• With or without ```uml in both
• live preview and read view