Call edit mode "Edit Mode"

This came up in yesterday's VDG meeting.

When we introduced the idea of an edit mode. We kind of danced around what it is, and didn't call it a mode, even though it is. We named the menu item "Customize layout". However in all of our communications with the user outside of the UI, we call it the global edit mode. Users themselves call it "edit mode". The code calls it "edit mode". We call it edit mode when we talk about it internally. I think it's pretty clear what it is in people's minds. So let's make the UI text reflect that.