Fixing the count problem on peer api, completed assessments.

Problem existed where you could:
1) Go in as Bob
2) Create submission
3) Go in as Tim
4) Create submission
5) Get Bob's submission for evaluation
6) Switch to Tim
7) Switch back to Bob
Even if you did not assess Tim (as Bob) the "completed" is "1 of 3". 

Now filtering the unfinished assessments. 

@wedaly @ormsbee @jrbl