FIX: Do not allow invite_only and enable_sso at the same time

This functionality was never supported but before the new review queue
it didn't have any errors. Now the combination of settings is prevented
and existing sites with sso enabled will be migrated to remove invite
only.

lib/validators/enable_invite_only_validator.rb

@@ -0,0 +1,14 @@
+class EnableInviteOnlyValidator
+  def initialize(opts = {})
+    @opts = opts
+  end
+
+  def valid_value?(val)
+    return true if val == 'f'
+    !SiteSetting.enable_sso?
+  end
+
+  def error_message
+    I18n.t('site_settings.errors.sso_invite_only')
+  end
+end