Add an image optimizer to the build pipeline

[MelissaAutumn]

We should add an extra step to the build pipeline to auto optimize jpgs, webp, and avif files as well as insert them via a jinja macro.

Shouldn't be too hard to implement with Pillow but there's bound to be something already built for this.

[MelissaAutumn]

I've added a script to do this, we should look into adding it into the pipline. https://github.com/thunderbird/thunderbird-website/blob/master/tools/compress_assets.py