Add support for AVIF images

[lapo-luchini]

We should add support for AVIF images (and maybe HEIF too? but I'll prioritize AVIF myself), such as this:

https://libre-software.net/image/avif-test/

I see two problems right now:

1. the bookmarklet doesn't detect it
2. if forced, the server fails on IntegrityError at /api/v2/pins/ NOT NULL constraint failed: django_images_image.height

[winkidney]

Would you please upload some AVIF test file here?
I may test it on my own instance and then update the code(bookmarketlet and backend) if necessary.

[lapo-luchini]

There are a few CC-BY-SA in this GitHub.

[Avatat]

I found out that Pinry relies on ImageField model field used to upload images and get some metadata about them: https://github.com/pinry/pinry/blob/master/django_images/models.py#L47

On the other hand, ImageField relies on PIL/Pillow to validate uploaded image extension and probably more: https://github.com/django/django/blob/main/django/core/validators.py#L572

And it turns out that AVIF support is waiting for two years to be merged to Pillow: python-pillow/Pillow#5201
While there's no native AVIF support in Pillow, we'll be forced to use some plugins and load them somehow in Django code, which is too much for my knowledge ;)