Process special file destinations

This should make the following changes to the build process, to mimic what Platform.sh does:

 * copy `settings.php` to www/sites/default for both profile & project mode (it used to be just in project mode)
 * symlink `<application>/settings*.php` to www/sites/default/
 * symlink `<application>/sites.php` to www/sites/
 * symlink `<application/favicon.ico` to www/
 * symlink `<application/robots.txt` to www/

Existing paths are deleted, with a warning message.