wagtailadmin.views.add_subpage view now sorts content types by their verbose names.

I noticed that adding a verbose_name to a model's metaclass changed what it was called on the "CREATE A PAGE IN XYZType" list, but didn't affect its sort order within that list. This was very confusing, so I dug into the code to find out why. The `wagtailadmin.views.add_subpage()` view was sorting the content types by their model name, rather than their verbose name, so I fixed that.