"Slug already exists" messaging not showing when entering matching value

[tw2113]

From this line:

let override_validation = (override) ? override.check : false;

Needs to be override.checked

[tw2113]

Also review/check history to see if we ever stopped saving of the content type if a match was attempted, as that doesn't seem to be the case right now.

[tw2113]

PR #1008