Harden abstractions

• remove antiquated abstractions/launchpad-integration
• abstractions/opencl-nvidia: don't allow PUx on nvidia-modprobe
• abstractions/private-files-strict: disallow access to the dirs of private files
• abstractions/private-files: disallow writes to thumbnailer dir (LP: #1788929)
• ubuntu-browsers.d/user-files: disallow access to the dirs of private files

Nominating launchpad-integration and opencl-nvidia for 2.13. Nominating private-files-strict, private-files and user-files for 2.10 and higher