lml/ost#347: Convert native confirm to dialog based confirm.

TODO:

* [x] Cache the settings at the session level.
* [x] Debug and fix the "Continue" button action.
* [x] Make `maybe_confirm` JS seamless.
* [x] Fix styles for checkbox and dialog.
* [ ] Add a simple page to view and change the settings.
* [x] Refactor and extract it into a gem / plugin.