Don't build with any default themes

Included themes are a decision that should be made by the
implementor of the platform, we can account for that in a later
change.
Include project drop-ins from the wp-content directory in build
Remove Twenty Twelve from the platform project
Remove Twenty Thirteen from the platform project
Remove P2 from the platform project
Remove the Spine parent theme from the platform project
Update .gitignore to reflect the theme changes
Remove WSUWP SSO Authentication from the platform project
Remove WSUWP Custom CSS from the platform project
Remove the WordPress Importer from the platform project
Remove Safe Redirect Manager from the platform project
Remove WSUWP builder from the platform project
Remove unignored plugins from our gitignore
Alter README - we don't use submodules anymore