Issue title: Feedback path: receiver is instructors: support specifying a max number of recipients
 Issue body: Current:
![image](https://user-images.githubusercontent.com/1673303/27067744-0fc824b8-503f-11e7-818d-72a330da8159.png)

Support something like this (screenshot from the case where recipient is 'other students in the course')
![image](https://user-images.githubusercontent.com/1673303/27067819-639c0aa0-503f-11e7-8bf7-6432bf2cc405.png)

Justification:
Requested by a user. His course has 30 tutors and each student gives feedback to one instructor only.

 Comments: 
 Comment 0: Can I work on this issue since nobody took it?
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