Less restrictive check if SVG is a diagram

Only use data-uri in preview. Partially fixes #40

Because data-uris are cross origin, our link fixing JS does not apply.
This uses data uris only in preview, fixing links for normal display.

We cou

Only use data-uri in preview. Partially fixes #40

Because data-uris are cross origin, our link fixing JS does not apply.
This uses data uris only in preview, fixing links for normal display.

We could think about fixing targets when saving already...

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Restore support for SVGs with a DOCTYPE

fix editing embedded diagrams

more refactoring for mediafile diagrams

more checks on the embedded SVG

we only want to work with diagrams.net created SVGs