FEATURE: Add support for Unicode usernames and group names

Co-authored-by: Joffrey JAFFEUX <j.jaffeux@gmail.com>

lib/validators/unicode_username_validator.rb

@@ -0,0 +1,13 @@
+class UnicodeUsernameValidator
+  def initialize(opts = {})
+    @opts = opts
+  end
+
+  def valid_value?(value)
+    @valid = SiteSetting.external_system_avatars_enabled || value == "f"
+  end
+
+  def error_message
+    I18n.t("site_settings.errors.unicode_usernames_avatars") if !@valid
+  end
+end