Add post duplication quick action

[zaguiini]

It seems like a useful feature (and one recent complaint from a client) to quickly duplicate a post so users can, for example, protect an existing version and work on a new draft.

It's possible to do it right now if they copy all the blocks and paste them on a new page, but it'd be quicker and easier to click somewhere instead.

I propose to add a new post quick action, as well as an action inside the post editor.

[SirLouen]

Duplicate of #31198.