Fix permissions check for interaction alerts

The code was checking :manage_alerts, when it should be checking
:manage_interaction_alerts instead.

Test plan:
1. Enable Alerts (beta) in Account Settings
2. Enable Manage Alerts for Teachers
3. With the old code, enroll a teacher into a course
4. As the teacher, go to course Settings > Alerts, then Add Alert
5. Upon save, "user not authorized to perform that action" will show
6. Now, repeat step 4 with the new code
7. The alert will be created successfully without any error
8. Still with the new code, disable Manage Alerts for Teachers
9. Repeat step 4 one more time (*)
10. The not authorized error will show; this is expected (*)

(*) This assumes the page from step 4 is still open, because after step
8, the Alerts tab will disappear.
Merge pull request #353 from sfu/fix-alerts

Fix permissions check for interaction alerts