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.
* [ ] Make `maybe_confirm` JS seamless.
* [ ] Fix styles for checkbox and dialog.
* [ ] Add a simple page to view and change the settings.
* [ ] Refactor and extract it into a gem / plugin.

@Dantemss - Thanks. Yeah.  I guess once I package this up into a gem, we can have reasonable expectation that no one is going to directly access the `mc_user_settings` table (or whatever we call it).  I'd definitely like to avoid hitting the DB once per request for this if I can.