Better PlantUML integration

[silverwind]

Feature Description

There are customization docs that instruct the user to insert some JS into their templates for PlantUML support. I think we can do better and add configuration options to set a URL to a PlantUML server instead and have our own JS handle the plumbing to get the graphs rendered.

Related: https://docs.gitlab.com/ee/administration/integration/plantuml.html
Also related (but not required): alecthomas/chroma#580

Ideally, the rendering would be done completely in JS without a dependency on a server but I don't think such a module exists (yet).

[lunny]

This may become a plugin after #34917 is implemented.