Grade edits in zero-weight categories change total grade

[glen3b]

See previous discussion in #25.

The editing of assignments in zero-weight categories in weighted gradebooks (which annoyingly don't display a (0%)) incorrectly affects total computed scores. This includes normal edits as well as calculating minimum grades for these assignments.

[aopell]

This is a strange case that is probably very uncommon. We should get around to this eventually, but it's of low priority.

[aopell]

That was an accident