Currently postponed/blocked on #1922454: Gut l(), fix theme('link'), have a working framework for consistently generated links inside and outside of twig..
Task
Use Twig instead of PHPTemplate
Twig sandbox: http://drupal.org/sandbox/pixelmord/1750250
Remaining
- Replace all theme functions and templates with .html.twig equivalent templates
- Add new preprocess functions for the .html.twig equivalent templates
- Update all hook_theme definitions