[pyrech]

The optimizer is only ever run locally when adding a new gif, so it has no reason to live in the bundle as a Symfony command.

• Replace GifOptimizerCommand and bin/optimizer.php by a castor optimize task calling gifsicle directly.
• Drop spatie/image-optimizer, the command tests and fixtures, and the gifsicle install step in CI.
• Add a test checking that every bundled gif is resized to 145px wide, and document in the README that running the task is mandatory when adding gifs.