Added grace_period, fixed a bug with extra late date and extensions, …

[etrickel]

Added @zardus suggested grace_period and fixed a bug with extensions and extra late date where a submission was not getting counted when it fell in between the two dates after an extension was added.

While working on those items, I also improved the due date messages on the module page. 1) If we are past the first due date but there's an extra late due date then it will show that date. 2) If both dates are passed, it will now display that the due dates have passed instead of just being blank. 3) If the creator has show_penalty set in the course.yml then it will show the current flag values (i.e., the penalty adjusted percentage).

[ConnorNelson]

I'm going to merge this now.

Going forward, the path to changing how grading works will be #586.

Ultimately, we have multiple instructors trying to use different grading schemes. Continuously increasing complexity in the dojo surrounding different grading paradigms / edge-cases is no good, and should be pretty nicely addressed by something like #586.