Generate thumbnails automatically from server

[seveibar]

• /snippets/download_image?snippet_name=...&image_content_type=pcb should return a thumbnail of the pcb using circuit-to-svg
• /snippets/download_image?snippet_name=...&image_content_type=schematic should return a thumbnail of the schematic using circuit-to-svg
• (elite) /snippets/download_image?snippet_name=...&image_content_type=model should return a thumbnail of the 3d model. This is the hardest by far and should be completed in a separate multi-step issue