Don't pester students with "this cannot be undone" message

[jpslav]

Currently, when a student works a problem they are required to "lock in" their free response before being shown an exercise's multiple choice options. The purpose of this stems from a cognitive science principle that it is better to have a student retrieve the answer from their brain before recognizing it in a list.

When a student submits their free response, we popup a simple browser popup saying something like "are you sure you want to lock in your answer? This cannot be undone". This is said over and over, even though the student probably gets the idea after working the first problem. After all, the button does say "Submit".

This task is to convert this warning message to a jquery dialog (we have existing infrastructure / helper methods for displaying them, see message_dialog and specified_dialog methods in the JqueryHelper). The dialog should have an checkbox with a "Don't show this message again option", and if they select it we should obviously not show it again.

We'll need to store the fact that they don't want to see this option again. I'm open to ideas on how to do this. I think because of the fact that we often log them out, we'll need to store this choice in the database instead of in the session. Storing whether or not to show one-time messages like this will likely come in handy in other "tour"-like settings where we want to convey helpful information to first-time users of different parts of our site. Hopefully, we'll be able to reuse the design for other sites (and possibly bundle in a gem).

cc @kjdav @Dantemss