Change the post_title database sanitization function towp_filter_kses()

[rachelbaker]

We should match WordPress Core's sanitization function when adding/updating the title of a Post. WordPress Core uses wp_filter_kses() but we use wp_filter_post_kses().

[westonruter]

I recommend against using wp_filter_post_kses() and wp_filter_kses() because they eat slashes. Consider:

echo wp_filter_kses( '\o/' ); // => "o/"
echo wp_filter_post_kses( '\o/' ); // => "o/"

The core function seems to assumes pre-slashed data but in my experience even with slashing it can be lossy. And since $request isn't pre-slashed, I think instead of calling wp_filter_kses() I suggest calling the the underlying code without the slashing:

$sanitized_post_title = wp_kses( $data, current_filter() );

I think current_filter() would be title_save_pre.

[kadamwhite]

In progress in #2840 but backslashes are still being removed after the proposed change. We need to dig deeper into how core is sanitizing these fields so we can better replicate that behavior.