Avoid use of wp_filter_post_kses to prevent lossy slash handling

[kadamwhite]

As discussed in slack following #2840, there are other places than #2788 where @westonruter's comments around the lossy slash handling in wp_filter_post_kses come into play; we should standardize these to use the underlying wp_kses functions.

Quoting @rachelbaker,

if we are not trusting the slash handling of wp_filter_post_kses() then we should do that consistently, and switch to wp_kses_post() or wp_kses() with the {filter_name}

[nylen]

This is being addressed at https://core.trac.wordpress.org/ticket/38609 along with unfiltered_html