Explicit button text in abandon dialog in all lang

The current post abandon dialog displays typical yes/no buttons and should
instead show more informative button text.

This commit *alters the structure* of the `post.abandon` node, and *adds
three new nodes* under it: `confirm`, `yes_value`, and `no_value`. The
current value of the `post.abandon` node is moved into
`post.abandon.confirm` for all languages. For English language text, the
nodes `post.abandon.yes_value` and `.no_value` are changed to `"Yes,
abandon"` and `"No, keep"`, respectively. For other languages, the
standard yes/no values are copied, allowing simple copyedits to make them
more explicit in the future.

I also make the necessary changes to the `composer_controller.js` file so
the new values are used correctly.