Add <none> placeholder to clear / unset inherited value

@damienmckenna just came back here for the same needs. The question is, do you think it will be realistic to implement the full featured override solution within the next decade? :D (Not asking you do do so or something like that, this is really meant to read with a smile).

My thought is the following:
I think needing to clear a predefined default in an override is clearly an edge-case.
It already works for any kind of string, it's just not possible to unset a value at higher level, because clearing the value would bring back the defaults. And entering a space would (hopefully) just set a space for the metatag value.

So if we'd allow a special string like
<unset>
<empty>
<none>
for technical users, we could quite simply extend the overriding to this remaining case.
And we could still convert that into checkboxes one day, if anyone solves it.

If you should change your opinion on this, our team would be happy to implement that, as we're running into this again and again.

Implementing the full-featured checkbox solution is great, but a big bunch of work and risky to never getting done.
This might be just "good enough" and fit things like Drupals magic "code> placehoder.

Reopening this as a simpler solution that won't involve greatly expanding the DOM structure and JS logic on edit forms.

So something like this might work, maybe?