FIX: Don't try to rename group when username is taken

FIX: Always rename groups with the default locale instead of using the user's locale

app/models/user.rb

@@ -220,7 +220,7 @@ class User < ActiveRecord::Base
   def self.username_available?(username, email = nil, allow_reserved_username: false)
     lower = username.downcase
     return false if !allow_reserved_username && reserved_username?(lower)
-    return true  if DB.exec(User::USERNAME_EXISTS_SQL, username: lower) == 0
+    return true  if !username_exists?(lower)
 
     # staged users can use the same username since they will take over the account
     email.present? && User.joins(:user_emails).exists?(staged: true, username_lower: lower, user_emails: { primary: true, email: email })
@@ -1271,6 +1271,10 @@ class User < ActiveRecord::Base
   WHERE lower(groups.name) = :username)
   SQL
 
+  def self.username_exists?(username_lower)
+    DB.exec(User::USERNAME_EXISTS_SQL, username: username_lower) > 0
+  end
+
   def username_validator
     username_format_validator || begin
       lower = username.downcase