Disable the "Report abuse" button if a user has already been reported

Hello,

I've implemented the feature request #2330. Here is what it looks like:

![report-abuse-button-disabled](https://gitlab.com/rymai/gitlab-ce/uploads/420d51906eac1c40c50701a0a340474f/report-abuse-button-disabled.png)

I hope that's an acceptable solution.

cc @DouweM

See merge request !1456