Split wp_set_terms_for_user for bulk/non-bulk contexts

[chrisvanpatten]

Fixes #19 and #20

This PR splits wp_set_terms_for_user into two functions:

• wp_set_terms_for_user contains the basic functionality, interacting with wp_*_object_terms and the term cache
• wp_bulk_set_terms_for_user wraps around wp_set_terms_for_user, with some extra help for handling admin editing contexts (bulk edit and on the user edit screen), such as capability checking, sanitizing keys, and pulling values from the $_POST global

wp_set_terms_for_user still supports the bulk parameter for backwards compatibility, although I did replace those with wp_bulk_set_terms_for_user in the plugin where appropriate.

[chrisvanpatten]

@jjj based on your comment in #19 if the sanitize step is removed (which makes sense to me) all that's left in the bulk-specific method is the capability check and pulling values from $_POST… and maybe, to be honest, those are better placed directly in the User_Taxonomy class than in a wrapper function?

Happy to rework if you think that makes sense.

[JJJ]

@chrisvanpatten I think you're right; that is probably a better place for that code to exist.

[chrisvanpatten]

@JJJ I think this is ready for review again!