- Comments: Prevent users who can not see a post from seeing comments on it.
- Shortcodes: Restrict media shortcode ajax to certain type.
- REST API: Ensure no-cache headers are sent when methods are overridden.
- REST API: Limit `search_columns` for users without `list_users`.
- Prevent unintended behavior when certain objects are unserialized.
Merges [56833], [56834], [56835], [56836], and [56838] to the 5.1 branch.
Props xknown, jorbin, joehoyle, timothyblynjacobs, peterwilsoncc, ehtis, tykoted, antpb, rmccue.
Built from https://develop.svn.wordpress.org/branches/5.1@56873
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