Add option for enabling SVG support for PlantUML

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Proposal

Due to this security vulnerability SVG support was removed for PlantUML. However, some customers were using SVG images with PlantUML. We should create a feature flag that would allow customers to re-enable SVG support.

Copying message from @robotmay_gitlab in slack:

On .com yeah it won't be coming back I suspect. It would be possible to bring it back for self-hosted but it would require reconfiguring their servers to ensure PlantUML isn't being served from a subfolder on the main GitLab domain, and we'd need to add a feature toggle for it I think