Replace gif.js with ffmpeg.wasm

[Pogodaanton]

Since gif.js hasn't seen major updates in its codebase for a while, its efficiency is to be desired. Like a greek god, ffmpeg.wasm rises from the ashes as the singular answer to the shrieking souls longing for an overkill multi-purpose client-side video processor.

Now it also supports GIFs.

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• Do a feasibility study. Is it any more efficient at encoding than gif.js?
• Can we implement better compression without compromising performance too much? The smaller the GIF, the merrier.
• Resolve licensing questions
• Implement ffmpeg.wasm in the GIF generator. The user should use Shadis without the generator logic loaded on default. Both the generation logic and the wasm has to be loaded asynchronously and solely on demand.