Do not provide a pre-configured PlantUML server by default

Currently, the PlantUML macro will work by default with http://www.plantuml.com/plantuml configured as the default PlantUML server.

Administrators may install this extension thinking that rendering is done solely client-side, without sending any information to a remote server. This causes a security risk as it could lead to the leak of confidential information.

In order to solve this, we would need to remove http://www.plantuml.com/plantuml as the default server configuration. The URL to this server can be provided as part of the hint of the server property.