[Modules] Next and previous buttons skip assignments outside availability dates

[Steven_S]

Problem: Each semester a few students are confused when they fall behind and the next button brings them to an assignment that is no longer available due to being past the "until" date.  Since "students must complete module activities in order," the next button does not let them click past the locked activity.  These activities are always (intentionally) the last activity in the module, so that students can continue if they miss that activity, however every semester has a few students who need to be specifically told to do that.  Some students speak up when they get confused, and other students fall further behind before I notice. 

Proposed solution:  Cause the next and previous buttons to skip past activities that are locked by availability dates