Improvements to Teachers' View

This adds managing Deadline Rule Deviations (personalised deadlines for students) to the teachers' view and also makes managing them in admin nice as well.

Also adds explanation to the "mystery box" in teacher's view.

Screen shot to show how adding deadline rule deviations in the view looks like.

![image](https://cloud.githubusercontent.com/assets/3489715/4531497/c0b0c466-4d8a-11e4-9b32-019ebf0f7d7f.png)