Use "danger button" for Reset Course Content

Due to the destructive nature of resetting course content, the red
"danger button" does a better job of reflecting just that.

Test Plan:
* Go to a course, click Settings, then Reset Course Content
* Verify that the confirmation button in the dialog is now red
Merge pull request #350 from sfu/fix-reset_danger_button

Use "danger button" for Reset Course Content