Rework processing of og:images in rich embeds.

[BurntimeX]

• Add an option to disable the loading of the og:image.
• Don't save the downloaded og:image in the filesystem.
• Instead generate a thumbnail of the og:image in Webp format, save it as a base64 encoded .bin file and embed the code directly into the rich embed.

ref https://www.woltlab.com/community/thread/310494-rich-embeds-ohne-bild/