ORA-37 Explicit action to begin turbo mode.

This is a small change to modify the way Continued Grading is initiated. 
See https://openedx.atlassian.net/browse/ORA-37

Initially, continued grading occurred when you expanded the peer assessment section after you had already completed your required grading. This became an issue as much sections of the page allowed collapse / expand. 

This change is to make continued grading more explicit by introducing a button. See the attached screen shot.

![image](https://cloud.githubusercontent.com/assets/235752/3457842/245fbfe2-01fc-11e4-89ba-0c5ae006099a.png)

@wedaly I did not add any tests; think there's something worthwhile in testing the change of initialization events in the javascript?
@marcotuts Could you take a look at the screen shot and see if this is alright from a UX perspective?
@srpearce The text on the button is new, just wanted to make sure this was logical.