Fix horizon line in Photos (as in iPhotos)
The edit tools are a little bit hidden, but actually easy to get to, once you know how.  

Choose your photo, double click to see it in single view, then just hit  Return ⏎  &amp; it will switch to edit mode. It's also in the Image Menu.

Rotate is unexpectedly not where to find the control, instead it's under Crop, highlighted on the right in this picture...