use nickname to link github account to user

My main email on github is different than the one my commits, so I had 2 accounts (one created when your site saw my commit and another one when I logged in with github). The only way to log in with the one with the tips was to use the link in the email. Otherwise I log in to an empty account.

With this change the user is found by his nickname, not his email. So it should solve the double account.

I won't be able to log in with the empty account anymore, but it doesn't really matters because it's useless.