Make slugs overrideable again

6589edb introduced a new issue that prevents post slugs from being
updated manually when in Draft mode.

The change was originally made b/c the slug wouldn't auto update when
changing Draft titles. This reverts 6589edb and corrects the original
issue in a different way, by registering an 'edit_form_after_title'
hook.

The issue originally existed because fix_editable_slug() attempted to
unregister itself if did_action( 'edit_form_after_title' ) was true,
but that action doesn't run unless something is hooked into it, which
is not always the case in 3.6. See
core.trac.wordpress.org/browser/trunk/wp-admin/edit-form-advanced.php#L4
53

Fixes #198