Remove config.php from git repository

config.php is currently in the repository which makes lots of things a pain. Certain actions (rebase) require a clean state, if phpBB is installed, config.php is marked as modified, thus requiring the use of git stash. When doing a hard reset, tracked files will be deleted, this is a major pain for config.php. You cannot do git add . because it would stage config.php (we've seen that before).

The solution: remove config.php from the repository, add it to gitignore. Might as well do the same with tests/test_config.php.

The build script needs to be adjusted to create a blank config.php.