Add thumbnailing support to media repository for SVGs (SYN-344)

[matrixbot]

This issue has been migrated from #1309.

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Python Image Library can't read SVG so the current PIL based thumbnailing code can't generate a thumbnail for an SVG.

For now we might just want to return the original image since it is already scalable.

(Imported from https://matrix.org/jira/browse/SYN-344)

(Reported by @NegativeMjark)

[ajkessel]

Is there an end-user (i.e. server admin) workaround for this issue currently? I'm using a Maubot plugin that sends urlpreviews to the room but results in a broken link when the preview has an SVG in it. I assume folks know about svglib which presumably could convert to a known format for thumbnailing.