Adobe Premiere Pro 360 / VR Tutorial #1: How to straighten or level the horizon in a 360 video

Tutorial on why and how to straighten or level the horizon in a 360 video (Adobe calls it "VR video") with Adobe Premiere Pro CC. No plug-ins required. This is NOT a tutorial for stabilizing a 360 video, which is a different topic. This is if your 360 camera was not vertical when recording and/or your 360 camera's vertical orientation sensor isn't working perfectly, and your horizon ends up tilted.