Fail gracefully when checking mapped cap against unregistered post type.
Post type objects are reponsible for mapping their capabilities to core caps.
As a result, when the post type is no longer registered, the caps are no
longer mapped. This causes problems when a post is left in the database after
the post type is no longer present, and WP does an 'edit_post' or other cap
check against it: a PHP notice is thrown, and the cap check always fails.
As a more graceful fallback, we map all post-type-dependent caps onto
'edit_others_posts', which allows highly privileged users to be able to
access orphaned content (such as comments belonging to disabled post types),
while minimizing the possibility of unintended privilege escalation.
We also add a _doing_it_wrong() notice, so that developers and site
administrators are aware that the cap mapping is failing in the absence of
the registered post type.
Props mitchoyoshitaka, DrewAPicture, imath, codeelite, boonebgorges, nofearinc, SergeyBiryukov, jorbin, dlh.