Add tick and cross for peer reviews

I have added tick and cross for peer reviews: 
A tick if the review is finished
A cross if it is not. 

It's way better to see the actual state of the review. 

I also had to remove the "cache" function, because of that the peer review state wasn't updated in "real time" (the cache must be re-created to see state changes). 

I'm not sure it is the correct way to solve this update problem. Let me know if there is a better one for your application. 

Test plan: 
* create a course with at least one assignment by activating the "peer_review" functionnality
* add two students to this course
* masquerade a student and fill in the assignment
* assign the peer_review to the other student
* masquerade the other student go to the assignment page to see the list of peer reviews assigned to him
* check that you see a cross because the peer review is not yet complete
* achieve the peer review
* check that you see a tick as the peer review is now finished - if you keep the cache functionnality, the tick won't appear untill the cache is updated...