Switched from 'username' to 'nickname' for anonymous logins.

This is a super minor thing but my users were having trouble with the "username" prompt for logging in. I would get emails asking "what's my username?" because they thought they had to have something official. Other users would reflexively enter their email address, which was both over-revealing and made the replies system unwieldy and confusing. I hope this change will cut down on those issues. 

Anyway, for my purposes it makes sense to switch to "nickname" instead of "username." I know this adds translation hassle and may not make sense for others. But I thought I'd let you guys make the call about whether this is helpful or not. Feel free to just close the request if you're not interested!