FEATURE: Mention @here to notify users in topic (#14900)

Use @here to mention all users that were allowed to topic directly or
through group, who liked topics or read the topic. Only first 10 users
will be notified.

lib/validators/not_username_validator.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+class NotUsernameValidator
+  def initialize(opts = {})
+    @opts = opts
+  end
+
+  def valid_value?(val)
+    val.blank? || !User.where(username: val).exists?
+  end
+
+  def error_message
+    I18n.t('site_settings.errors.valid_username')
+  end
+end