Adding owner/moderator clears the other form box

This is sort of a silly bug, but I'm recording it because it may be a beginner-suitable task.

Reproduce as follows:

1. Go to the list members page for a list.
2. Enter something in the "Add Owner" form.
3. Enter a valid email address in the "Add Moderator" form and submit it.
4. The content in "Add Moderator" form will stick around, but the content in the "Add Owner" field will disappear.

This will behave the same way if you use the opposite field (e.g. clicking "Add Owner" will clear anything in "Add Moderator").

Probably we want either both fields to stay filled or both to disappear, although one could make a case for preserving the contents of the one not submitted and removing the content of the one just submitted since, presuming it was successful, there will be no point in submitting the same data again. Pretty much the only behaviour that doesn't make sense is the one we have right now.