Sign up is broken if username lowercased is taken

[Karen Chan]

Log taken from when Ed tries to login using his google account:

  User Load (1.1ms)  SELECT "users".* FROM "users" WHERE "users"."username" = 'EdWoodward'
CreateUser#exec user: {"created_at":null,"first_name":"Ed","full_name":"Ed Woodward","id":null,"is_administrator":false,"is_temp":true,"last_name":"Woodward","person_id":null,"title":null,"updated_at":null,"username":"EdWoodward","uuid":null}
  User Exists (1.1ms)  SELECT 1 AS one FROM "users" WHERE "users"."username" = 'EdWoodward' LIMIT 1
  User Exists (1.7ms)  SELECT 1 AS one FROM "users" WHERE LOWER("users"."username") = LOWER('EdWoodward') LIMIT 1
CreateUser#exec user.errors: ["Username has already been taken"]

From #123