Handle "duplicated" nicknames for signup more user friendly.

[salzig]

Users with the same nickname on twitter and GitHub have a hard time signing up using GitHub when they had an account using twitter already.

for sake of situation description I'll use the nickname "example"

• User signed up years ago using Twitter with his nickname "example"
• Twitter "crashed" our auth integration
• User is unable to authenticate and thereby accessing his old account.
• User tries to setup a new account using Github, where he/she uses the same nickname "example"
• User can't create a new account, cause on_ruby complaints about already taken nickname.

Possible solution: Allow the user to change/alter his nickname while signup.

[phoet]

@salzig has this been handled with the recent changes?