Unify patch system (in Dockerfiles)

[nightkr]

Currently, each product has its own copy of our overall patch system. It would be nice to unify them into one implementation in one location that all of them can reference.

[lfrancke]

I'm not sure how much we want this issue to spiral out of control but we can also use this to discuss how we want to handle patches and build-from-source in the first place.

We can also fork the upstream repos and build the things including patches there once, push them to a registry and only fetch them from there during image build....

[nightkr]

#994 switched our development workflow to using the new Patchable tool, we might want to add a mode to it for applying them in a build too.

[dervoeti]

Implemented by #1032

The apply_patches.sh scripts are now replaced by patchable checkout, which checks out the source repository (defined in the patchable.toml files) and applies our custom patches on top of it.

[lfrancke]

⚱ RIP apply_patches.sh we hardly knew ye!