Convert assets GIFs to a smaller format or ban them

Assets page GIFs take around 16mb (at the time of writing this) and unfortunately don't get optimized by Zola `resize_image` (see issue #339 & PR #360). Ideally these should be resized to fit in `340x340` (current assets thumbnail size) and converted to a better image format (e.g. webp, gifv, mpeg…)

Ideally this should be automated.
- `ffmepg` could be used as it supports gif => webp.
- or, maybe there is some crate that supports this conversion and can be integrated with the assets generation code