WIP: rename nets that contain slashes

The templates for all Arduino shields have a couple of nets, whose names contain forward slashes. When trying to add a net name in Eeschema and selecting one of those nets, one gets a validation conflict saying that the net is invalid. So the template isn't usable out-of the box.

I tried to fix it for the Arduino Uno template, replacing the slashes with underscores and doing forward annotation. This appears to work insofar as the error message no longer appears.

I am willing to do the other Arduino templates as well, but I don't know if what I did is correct and complete. A bunch of seemingly unrelated changes also happened in the files. So I'd be thankful if someone who knows what they are doing could look over this first.