Duplicate reviews for a request

[dmarcoux]

Issue Description

It's possible to add a duplicate review for a request. For example, if I added a review for the user Iggy, I can add another review for him again.

Expected Result

I expect this to not be possible, unless the previous review isn't open anymore (it was approved/rejected).

How to Reproduce

1. Go to a request overview page for which you can add reviews.
2. Click on Add a review
3. Add a review for a user
4. Add it again
5. See the duplicate reviews

Further Information

[coolo]

unfortunately there is a requirement from (previous) customers to have that. And we even have a test case to avoid regressing this: test_create_request_and_multiple_reviews

[coolo]

I don't think the webui needs to support this - but abstain from thinking in terms of model validations :)