Introduce `meta_box_sanitize_cb` taxonomy argument.

The `meta_box_cb` argument was introduced in [25572] to allow plugin
authors to provide a custom callback for rendering their taxonomy's meta
box on the post edit screen. However, the routine used to handle the saving
of these custom taxonomy meta boxes was not customizable, but was instead
based simply on whether the taxonomy was hierarchicaly. See [13535].

The new `meta_box_sanitize_cb` argument defaults to the "tag" routine for
non-hierarchical taxonomies and the "category" routine for hierarchical ones,
thereby maintaining the current default behavior. Developers can override this
when the data passed from their `meta_box_cb` differs.

Props boonebgorges, ZaneMatthew, stephenharris.
Fixes #36514.
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git-svn-id: https://core.svn.wordpress.org/trunk@42040 1a063a9b-81f0-0310-95a4-ce76da25c4cd