[v1.78.0] Better AVIF support

Looks like AVIF is here to stay at Lemmy so I'm going to try to address it...

Changes so far

• Add setting to save images/videos in subfolders based on the community. Eg. an image from c/cats will be saved to Summit/cats. Default: off.
• Fix an error that can occur when signing into a Piefed instance.
• Add experimental AVIF support (again)