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, otherwise there should be no net visible change.

I also make the necessary changes to the `composer_controller.js` file so the new values are used correctly.

All changed language files pass yamllint.com checks. English was tested directly, but **none of the other languages were confirmed to work**.

![image](https://f.cloud.github.com/assets/133882/2300636/cdad5f32-a0fe-11e3-95d9-57f46faf91ff.png)