feat: abstract cache

[GeopJr]

:warning: WIP

Mostly discussed on #954

Bringing back the classic in-memory cache.

Benefits:

• Avoid creating multiple paintables of the same image, lowering ram usage
• Avoid processing already processed images

TODO:

• Add a "dont cache" option. We really don't want to cache profile pics/banners. They change but the url stays the same, let libsoup handle their caching
• Combine with #915
• Increase maintenance timeout to probably 1 minute?
• Use url hashes instead (same as #915)
• More testing on the media viewer... maybe let libsoup handle that?
• A way to update paintables so race conditions / updates don't break existing ones? Or maybe not inserting if it exists already?