eslint: configure import sorting rules

[jelly]

This more or less enforces existing practice, but cleans up many places where we've deviated from that. We sort into these groups:

• 'react', if present, always first
• external (node_modules/) imports
• cockpitlib (pkg/lib/) imports
• parent (../) imports
• sibling (./) imports

[martinpitt]

Still some complaints about build.js

[jelly]

Still some complaints about build.js

Yeah, npm run eslint does not lint build.js which makes sense but then fails to autofix :)