Add toggle to indicate that a user is trying to migrate content types into CPTUI

[tw2113]

Right now we prevent saving of content types where the slug is already registered elsewhere, whether it be a different post type, page slug, etc.

Let's offer a checkbox that essentially indicates "I'm trying to migrate things in to CPTUI, let me save this".

The checked value should only be used to allow saving anyway, and not reject. I believe we have a filter on this, but I'd need to double check.

The checkbox should not save a value anywhere, so we do not have a new option to save. Just needs to be used temporarily on save.

[tw2113]

Original idea source: https://wordpress.org/support/topic/how-register-existing-cpt-in-plugin/

[tw2113]

Merged in to release1150 branch