Change note length validation to ignore mention domains and URLs (#9717)

Fix #4419

app/validators/note_length_validator.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+class NoteLengthValidator < ActiveModel::EachValidator
+  def validate_each(record, attribute, value)
+    record.errors.add(attribute, I18n.t('statuses.over_character_limit', max: options[:maximum])) if too_long?(value)
+  end
+
+  private
+
+  def too_long?(value)
+    countable_text(value).mb_chars.grapheme_length > options[:maximum]
+  end
+
+  def countable_text(value)
+    return '' if value.nil?
+
+    value.dup.tap do |new_text|
+      new_text.gsub!(FetchLinkCardService::URL_PATTERN, 'x' * 23)
+      new_text.gsub!(Account::MENTION_RE, '@\2')
+    end
+  end
+end