Post abandon dialog: add explicit confirm button text

The abandon post dialog uses plain Yes/No default button text. Instead, each button should say what it does more explicitly.

This text hasn't been changed since the initial release of Discourse (though it has moved around a bit).

The change passes yamllint.com checks, and follows the style found in the rest of the document, but I don't know if this change actually has the desired effect and I have no means to test it.